Unlike other RPGs I don't see that you need a thief... you don't need to pick locks etc as far as I can see. However...
A mage seems to be difficult to run more than one because of the amount of clicking in combat. While a thief can attack with melee from the back row.
Because of that I think 2 front row fighters, a mage and a thief is the right combination.
I think one minotaur is the right amount too, because while I like the idea of humans getting more skillpoints, I also notice that minos get a bonus for having a skull. I assume that means there will be a number of skulls in the game. It makes sense to have a character that can use them, while it doesn't make sense to have two minotaurs because if they really are balanced against humans for a front row fighter then that'll also be balanced with skulls taken into account.
As for skill advances, I'm going for armour for my front row guys rather than damage, because there's damage coming from all 4 characters and my back row guys can't take a beating.
I'm going for generic skills rather than specific weapon/magic type. I suspect there will be one powerful sword and one powerful mace later on, I don't expect the two most powerful weapons will both be the same skill type. I also suspect there will be a goto spell, and I don't know what it relies on..
So I think initially I'll be doing armour/armour/assassination/spellcraft on my skills.