I'm really happy with the acquisition of Muhammad, but I don't think it warrants trading down in the draft. There is plenty of talent to pick up, whether it be a decent RB, an O-lineman (whether they warrant fourth pick money or not), a LB (albeit this is a weak year for LBs) or a DB.
Muhammad is pretty good and will immediately become the Bears' #1 WR, but at 32 his longevity is in question. The Bears could stay at #4 and pick up Braylon Edwards
or some other decent young prospect and build for the future while they have the luxury of playing with a solid starter. The past few seasons they've been trying to build and build without much depth behind them.