yes, the de that uses more space is kde4 followed closely by gnome, around 2gbs on / partition when installed
those will barely fit on a 4 g usb stick, only 3.7 gbs of space for the user will leave you with practically no space for home and for swap(swap on a usb stick is painfully slow but if you have lots of ram this doesn't matter really)
if possible, get a fast 8 or 16 gb stick, 8 gb sticks are very cheap now, i just bought one of this in 8 gb size(7.4 gbs for user)
http://www.kingston.com/flash/dt100g2.asp?id=2that way you can have a / partition and home partition(or not, more simple not to) in a better size, without worries of have problems with space quickly
the minimum installation already contains firefox, what you have to add is wine and that doesn't use much space, 40 mbs more or less