I 4-box with Guardian, Templar, Warlock and Illusionist.

I read a post here mentioning not to take a bard for a 4-man group, as they benefit larger groups more. A 4-man group is just the trinity 3-man group of Tank, Healer and DPS with 1 extra to help out in taking on either larger pulls or speeding up boss kills.

A DPS/CC class would fill this roll.

But for the 4th pick, you really can't go wrong, as long as it can DPS.