I can't really add much to what Andy's said there although one thing I might add is that a few other motors, such as Magmotors and LEMs, are for their power actually lighter than the Bosches - check this sort of still relevant table I did a while ago for figures - and if you're really stuck for weight switching to them may help lose a kilogram or two.

Really for four wheel drive you don't need four motors - the only recent heavyweight machine I can think of that's used four motors is Mr. Nasty, which though powerful had to be very compact and forego active weapons to use that (and it was never particularly successful in competition).

I'm hoping to trim my team down slightly and possibly update a few vapourbots in 2011 myself - if everyone else's doing so I might as well join in, and in a few cases it's long overdue