I thought you need higher levels of find trap to find and disarm traps in later game, as well as other thief skills - what is the meaning of spending points in thief skills if it is suficient to have thief on 4 level ?

What about diference between 6 and 4 guys party ? What do you think guys makes 4 characters party diference in exps ?

Read Hickory's full comment. Imoen will need some potion/spell/item help later in the game to disarm/open everything you run across, but there are sufficient resources available for her to handle it. An invisibility spell can let her handle scouting, removing the need for lots of Hide and Move Silently (though items are available to boost those as well).

A 4 man party will have fewer resources for dealing with challenges. They'll have fewer actions per round of combat, fewer people to divide the enemy attention, fewer characters that can soak up some damage, fewer spells, fewer classes to provide a variety of abilities, etc.

However, the 4 man party will gain more experience per encounter, and will level faster. This means they'll reach the higher tiers of ability sooner than a 6 man party, and some of their abilities will be stronger than a 6 man party who had completed the same amount of the game. [Edit: The experience per encounter is split among fewer people in a 4 man party. I'm not saying that a 4 man party gets more experience overall, just that there are fewer people sharing the same amount of XP.]

I don't know how much of a level advantage the 4 man party will have over time. I like taking a 6 person party to have extra bodies doing things and to have more NPC interactions available. [Edit: Also, a 6 man party can use more of the loot you find in your adventure.]

Both sizes of party will be able to beat the game. You can take anything from 1 character (provided it can advance as a mage and/or thief) up to 6 characters all the way through ToB.

[Edit: Each person in a 4 man party will probably get about 1.5x the amount of experience that a person in a 6 man party would have. This excludes quest rewards, where a certain amount of XP is given to each character in the party regardless of size.]

I wasn't saying that Dispel Magic would work well as a universal trap removal; I'm just saying that, when dealing with traps that are magical, that particular spell ought to come in handy. Meanwhile, Detect Magic can be useful for detecting magical traps.

Against non-magical traps, one would need to find a different method; this isn't a universal method of dealing with traps. Perhaps the party's barbarian could fly into a range, smash the trap mechanism, and get around the trap that way; also, if the party has a healer, the barbarian can afford to take damage, which the cleric can then heal. (Of course, this would require that healing magic be at least decent in the game.)

The way I see it, if the game is going to have traps, there shouldn't be a universal method for dealing with them.

I can categorize thief skills into two categories: "essential" and "non-essential".

The essential thief skills are those involving locks and traps. These skills can be easily boosted with potions, or if you have some way to boost the character's Dexterity. At high levels, you could even use Wish to (hopefully) boost your stats to maximum long enough to deal with the troublesome trap. Alternatively, you can opt to just trigger the trap and either tank the damage or find a way to avoid it (Mirror Image can protect against damage effects, for example). It just so happens that Imoen, in particular, has most of her skill points spent on the essential skills.

The non-essential skills are those that are nice to have, but that you can do without (and don't necessary need a substitute). Setting traps, while a valid strategy, is not required; hiding is never necessary; detect illusion, probably the most important of these skills, can be trivially replaced with True Sight/Seeing (at least if not using a difficulty-increasing mod). There are a few nice items you can pickpocket, but you don't need them, and I am not sure if any of them are actually unique.

Also, if you have multiple thieves in the party, they can focus on different skills; hiding is probably the only skill that is worth having on multiple thieves.

There are three cases here:

At low levels, when XP requirements scale exponentially, a party of 4 will likely be at most 1 level ahead of a party of 6, and probably none much of the time.

At high levels, when XP requirements are the same for every level, a party of 4 will gain levels 50% more often than a party of 6. In particular, the mage in the party of 4 will get 9th level spells before the one in the party of 6 gets 8th level spells, if we ignore non-split quest XP.

At really high levels, levels stop being worth as much, and you eventually reach the XP cap. This is more likely to happen with a small party, and will happen sooner the smaller the party. At this point, the smaller party loses much of the XP advantage that they used to have.

There's another advantage to smaller parties, which becomes especially apparent in speedruns; many area transitions require the whole party to be close to the exit. When you have a limited number of Boots of Speed, it is much faster to get to the next area if your party is small enough for everyone to get one. (Of course, you can use Haste, which makes everyone nearby fast, which makes this fact less important in the classic edition; apparently in the EE Haste *stacks* with the Boots of Speed, so it again becomes important to not have slower characters.)

Wouldnt it be easier simply to pick good thief other than Imoen ? :D But yeah you are right there are some other ways to run the game without a good thief, I never tried them though.

In BG2, not really, the selection of recruitable thieves is rather sparse, and not everyone wants to use the main character slot on a thief. In particular, you have:
Nalia: Worse than Imoen as a thief.
Jan Jansen: Can level up as a thief (unlike Imoen and Nalia), but has horrible (IMO) voice acting, and still levels up more slowly than a single class thief would. Probably your only decent option, however.
Yoshimo: A pure thief (albeit with a kit), but leaves right when Imoen (Spellhold version) joins; skipping the event requires skipping a sizeable portion of the game (most of Chapter 4, including many critical plot events). There's a way to get him later (at least in the original version), but it requires being fast and still means you can't use him in Chapters 4 and 5, and is clearly not intended by the developers. (Also, importing an incomplete save into ToB will cause him to be removed from the party if present.)
Tutorial Imoen: Only way (without console cheats) to take her out of the tutorial is to import a tutorial save into ToB (skipping SoA entirely), so you can't use her in SoA, but at least she's good at setting traps.

The only other options are to make the main character a thief (something not everyone will want to do), creating a generic thief using multiplayer trickery, mod the game to add a recruitable thief, finding an NPC thief to use the Ctrl-Q cheat on (no guarantee that won't cause issues later), or, in the EE, recruit a certain new thief who happens to be evil (and hence unsuitable for many parties).

So really, BG2 is rather sparse in terms of thief selection.

There is a reason the find traps spell disarms traps in NWN... and Thieves can't disarm Spell wards... so... I guess you're both wrong.

How true that is. I once had a party frothing after an elderly Halfling woman absconded with nearly all of the 500 gold the party had started with. Replaced it with lead, she did. The funny thing is they never questioned why they had started at level 1 with such a high amount. By the time they discovered the theft they had about 25 gold left, bwah ha haha!

Flynn

Imoen is a mage in BG2, that has some bonus thief skills. This has been said numerous times.

If you want the HLAs of a thief, you gotta go with Jan or play the PC as a thief. BG2 is lacking in the NPC thief department. The only single class thief is Yoshimo, and we all know what happens with him.

That being said, Ive never found backstabbing to be essential, nor removing illusions, nor picking pockets. I'd rather have a second spellcaster.

If you desperatly want to remove illusions, pick up Keldorn.

Backstabbing is not an issue, this is not Icewind Daler.
Consumables are never a problem from the point of BG2. I see you want stealthing though, and that's related to the backstabbing issue. Well, get the Staff of Rogue Stone Doorway, and your mage will overplay any thief.

There's 1or 2 item worth stealing (ring of regenerationand bottle of efretii), otherwise you only can use the skill to get questrewards without doing the quests.

And if you desperatly want to use traps HLA you don't need a thief, as those traps work 100% success rate. Bards also can lay them. And bards at least marginaly are better fighters/overal characters than thieves.

EDIT: oh,and you don't want to brew potions and scirbe scrolls, believe me.