I'm playing thru with 4 characters in HoW from the beginning. I find that 4 is about right in that my magely characters (cleric/mage and bard) are finding spells about the same time they are reaching the spell level to cast them.
Bards are super cheesey at level 9 in HOW cuz they get a new bard song that "entrhrals" the enemy (hold person) for a turn and has an area of affect of about... 50 feet? it's pretty aweseome to see a horde of 9 yaun-ti slither towards you, then stop in their tracks so they can listen to pretty music.

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Make sure the bard is Good of some type, so he can get the benefit of some of the "good only" items like the 'shimmering sash' (HoW, at Orrick's)
Also, you want an evil cleric so you can take advantage of the cool new evil clerical spells (cloud of pestilance is neat). Make your tank a good guy too (half-elvin ranger/clerics are the best) so they can use the new maces too.
In the old IWD, there is a somewhat cheesy mace for non-good characters that gives life and a point of strength 10% of the time. How has an even cheesier mace that lowers enemy luck by 4. there's nothing finer that beating extra-planar 6 armed ladies who get a -8 to their hit (i hit her twice).
Fighter mages can be powerful, but there is that whole armor issue. make sure they have a high dex and con at least and you might get by with robes and morror immage/stone skin.
Dual-classing: remember to dual class BEFORE level 9(name level) if you want to make sure you get all the bonuses from the former class. Example: a theif becomes a fighter at level 8. when the fighter reaches level 9, he gets all the bonuses of a fighter of tha level includiung extra hit points for high Con up to and including 9th level then getting a hp bonus for high con for each level after that. HOWEVER, if the theif waits until 9th level to dual class (for the backstab modifyer?) then the 10th level fighter will NOT recieve the con bonuses and you character will be forever stuck at a +2 hp per level bonus, even though they may be a fighter with high con.... does that make sense?
though some may disagree, i assert that Icewind Dale IS a Cleric's game, and having two of them in your party (hopefully dual classed) is a big advantage. sometimes you NEED to send a healer up to your tank to throw hitpoints at him as they beat on the brats.
Though i'm not a huge fan, the fighter druid seems to be favored by IWD too in that there are some druid only items and spells (even ranger/clerics don;t get ALL druid spells in HoW)