I've had a lot of luck with the following setup, or some slight mutation of it:
Member 1: Human Soldier, Elite Warrior trait, point emphasis on strength, endurance, melee weapons, defense. Secondary emphasis on dexterity, hardiness, and thrown weapons. Basically this guy's the "tank."
Member 2: Rebel (Nephil works rather well here later in the series, as does a Slith), Nimble Fingers trait, point emphasis on dexterity, ranged weapons (bows), tool use (for unlocking things and disarming traps). Secondary emphasis on melee weapons or polearms (personal choice of backup weapon. Polearms do a lot of damage but are usually 2-handed), strength, and endurance. This guy's your "rogue" character.
Member 3: Human Priest, point emphasis on intelligence, priest spells, first aid. Try to keep them out of melee range. Priest spells aren't affected by armor so feel free to suit this character up as much as you want.
Member 4: Human Mage, Natural Mage trait, point emphasis on intelligence, mage spells, arcane lore. Since mage spells are affected by armor (as in "don't let this one wear anything but robes, for maximum effectiveness") and this guy will be very frail, keep him away from the fray as much as possible, and have him blast the enemy with spells from as far away as possible.
General notes: Potion making is a pretty handy skill to have, and should be focused on either of your magic-using characters (but not both, as it's a personal skill). Nature/cave lore is also fairly handy, but that's taken across your entire party so you can spread it across all your characters and get the bonuses.
Please note that even with all this info, the game's still going to be difficult. They're definitely of the old-school. Then again, the Exile series (what Avernum 1-3 and Blades of Avernum are remakes of) was even harder, and those had parties of up to 6 characters. So just be careful, plan things out, and save often.