Is there a way to create a balanced party of four (with which you can finish the game) without having to worry too much about complex multi-class strategies, racial bonuses and penalties, powergaming, etc... ???

I play on a LAN with a friend of mine and we just finished IWD1 with 6 characters. We're gonna start IWD2 now, but we would like to play it with 4 characters, because of the obvious advantages (levelling faster, less switching between characters, ...)
I read Jukka's UPP, but we don't want to play IWD2 with a mouse in one hand and a game guide in the other...

Ok, to resume... Is there a party of 4, that has all the abilities you need to finish the game, that doesn't rely on powergaming...? What characters should we take and how should we let them evolve...?

The game is made in a way that you can finish whatever your party is (maybe a bit hard with an all-rouge-party with badly placed ability-scores.) A good 4-p party should have one at least almost pure-classed arcane caster, a high level divine-caster and someone with meleeing power (I usually combine the later two, having a strong divinecaster as tank). The other one or two characters is up to you. If you like playing with a decoy taking the hits use jukkas decoys or make a Deep Gnome Monk with maxed dex and wis.
Your party should look like this:

1:Decoy or whatever
2: Strong Meleecapable cleric (or druid)
3:whatever you like
4: Sorc or Wiz

Ok, thanks for the quick reply...

We already figured out that the cleric is one of the most important characters of the party... Basic idea is healing & fighting, right? But what's the best option: taking a pure class fighting-cleric (Tempus) to get to high level healing spells; or taking a multi-class healing-cleric, with a few levels of fighter...?

I would take a pure cleric (Morninglord, stormlord or dreadmaster) and add maybe one lvl fighter,barb,ranger or pal (depending on race and preferance) to be able to use other weapons (maces are good, but if wou want him to be a tank you'll probably want him to be able to use slashing weapons too). The Battleguard don't have any good domain-spells and 2 points in axes aren't much better than one (the difference is +1 to hit). When you've got enough lvls to cast all the spells you like you can take more fighter-lvls (4 for WS, but i usually just have one). The Morninglord and stormlord has the best domain-spells and my favorite the dreadmaster gets a +2 to wis in a quest later in the game (ch5). I usually make him a dwarf (good tank), DG (good AC, but problems fith favored class) or drow (spell recistance and favored class for female-drow) but almost every race can be a good cleric since no one have penalty to wis. Just make sure that the favored class don't hinder your multiclass-plans. If it does (= FC is not any, cleric or any fighter class) you should stay pure and later on spend one feat on a good slashing weapon (preferably axes as they come in all sizes).

I think the game manuals suggest:
A warrior, a rogue, a mage and a healer

I'm probably not a good suggestor for parties as I tend to go pure class instead of multiclass as some people prefer to do.

A cleric is quite useful and as IWD2 enables spontaneous casting a cleric with Good alignment or Neutral with a Good god can cast healing spells from memorised spells which aren't that useful. IIRC the druids cannot do this. I would go for a pure cleric for the maximum amount of spells you can get, Give them a good str and con to hold their own and increase their wisdom a lot if you wish to get the maximum amount of power behind their healing and spells. Cha is okay if you want more turn undead but you have feats that help with that. Druids are good characters I find and fun to play, but I think the Cleric might be more useful as your healer, could multi class if you really wanted I guess.

Mages: Sorcerers, Wizards or Bards. Wouldn't go for bard as the main arcane caster of the party. The choice between Wizard and Sorcerer is simply the choice between intelligence or charisma and tactics or versatility. You can find quite a few scrolls in the game for the wizards too for quick use.

Rogues are pretty explanatory, Bards don't do too well as the main thief of the party. So a dexterous rogue is always useful.

Warrior: Fighter, Barbarian or Paladin if you would like a challenge. Fighters offer the best range of feats and amount of them to use, good for powering up quickly in the chosen weapons and styles. Barbarians get the ever useful rage and speed. Paladins are quite fun to play with a few spells to enhance their abilities and the turning undead. Rangers could also be included I think as they are like combat orientated druids really with freer alignment.

Try to pick a base alignment for your party then think if you want more power through spells or sheer force then pick the classes around that.

As 8peole said it's really useful to use spontaneous casting so make sure your cleric isn't evil. A fulltime-rogue is the worse class in this game as there aren't many powerful traps and not much to pick-pocket. The sneak-attack is only worth it if you really like micro-maneging as it's not always easy to sneak up behind the enemies without them noticing you and you'll die quickly if they do. If you have a wizard you can give him some points in search and remove traps but you can beat the game without. You should even consider giving him a lvl of rogue as a starting-lvl to get enough skillpoints to cover everything you like (this makes the wizard better for all-round purposes than the sorc but the sorc is more powerful caster). Just don't make a full-time rogue if your not a fan of sneakattacks.

Due to its challenge ratings structure, IWD II is best enjoyed with as few charecters as possible.
A competent party should have:
1> A frontline charecter capable of high melee damage.
2> A high AC charecter, generally a DG.
3> A damage focused arcane spell caster.
The fourth charecter can be used to complete the party in a manner you find suitable.

1> Barbarians and Clerics both make great melee chars. The rage is a powerful tool and only gets better as you progress along. Spell buffs can also compensate for the lack of fighting abilities, although that is only temporary.
The fighter is only good enough for a few levels to take the additional feats imo. The advantages of Weapon Specialisation can be over whelmed by spells such as Champion's Strength or the Barbarian Rage when necessary.
Paladins are too specialised and there is no way I can RP one.
Rangers as a pure/main class is not even worth discussing.

2> LE Deep Gnome Monks manage to make their way into practically every party I have made lately. They can be one of the most versatile charecters you have. The easiest way I have found to play one is "the confused gnome". Give him a level of practically every class possible. A Monk[1]/Ranger[1]/Fighter[2]/Rouge[1]/Cleric[1]/Illusionist[x] may sound weird, but trust me, it works :D
Enough Int to take all important skills lets the others get on with their job. Just remember that his job is to act as a sheild not kill, that you leave to the first guy.

3> A Sorcerer. No doubt about it. No other choice either. You will always find that there is a spell your wizard cant cast since you still have not found a scroll to scribe. A casting of 'Eagle's Splendor' will increase your DC for a particular fight, something no other class can boast of. You can also add Paladin levels to increase your saves, or make him a Drow to give him Spell Resistance.

The fourth charecter is really there to help out where you feel you need some help.
Unlike others here I dont think healing is so important that you need a full time Cleric. Still whether you pick a spellcasting Cleric, a shapeshifting Druid, a singing Bard or a loony Monk is your decision. Go with what you are comfortable with.

Here is my favorite party.
Human Battlegaurd of Tempus[8]/Barbarian[x]Fighter[2]
Deep Gnome Rouge[2]/Ranger[1]/Fighter[2]/Demarch of Mask[1]/Monk[1]/Illusionist[x]
Aasimar Paladin of Ilmater[3]/Sorcerer[x]
Drow Bard[11]/Druid[x]

Just of note, if you’re going with a 4 person party do not take a wizard, unless you plan on it being a deep gnome (ECL 3). Shrikant hinted at this, as the game has some balancing issues with scrolls later in the game. Your Wizard will level up and be able to cast 5th level spells, but there are none available. This will happen for all the higher level spells too. A Sorcerer is the way to go in IWD2. The only way around this is to have a 6 person party and make your wizard a ECL penalty race like drow (won’t work in a 4 person party).

Anyway, I would go with:

Fighter – lots of feats make him powerful. A Barbarian also works.

Cleric – you need the healing, plus he can fight. Choose Dreadmaster because of what others have said above.

Make a Rogure who is multi-classed as a fighter (sorry, but you probably need one mult-classed character), barbarian or Ranger. You need a rogue for the traps, etc… But you really don’t need a pure classed Rogue, so you might as well make him into a tank. A Ranger would be helpful to get Wilderness lore, which you need and a lot of the Ranger skills are also Rogue Skills. But a Ranger is kinda weak, so you might want a fighter for the feats.

A Sorcerer. With a 4 person party a Drow is an excellent choice. You’ll be casting 9th level spells by the end of the game. But really many classes excel at the sorcerer role, even a halfling.

Battleguard of Tempus
Monk[1]/Druid[x]
Paladin[1-2]/Sorcerer[x]

This is what I am working with now. If you want another char, add a pure fighter or barbarian.

Hey, thanks everyone for your advice... We took a bit of everyone's advice, added a bit of our own taste, and came up with a very good party... We've got a human fighter (maybe add a few barb levels later on), a thief (add fighter levels when enough thieving skills), a shield dwarf cleric (dreadmaster) (maybe + 2 or 4 fighter levels) and a pure drow sorcerer...
We rushed through the prologue and chapter one in one evening so... In our opinion it's going too well, but that's probably because the first chapters are not so hard, correct?