If I read your post correctly, you want to swap a Bushmaster barrel onto a Colt M4 upper receiver. If so, then the barrel extension on the Bushmaster barrel will not have the enlarged, enhanced M4 feed ramps like the Colt. You can still install the Bushmaster barrel, but the feed ramps will not match the cuts in the upper receiver itself. For semi-automatic use, I imagine this would mostly be a cosmetic issue and have little effect on how well the weapon functions.
To answer your question, though, yes, you would have to have the feed ramps of the Bushmaster barrel extension properly machined to get the benefit of the M4 style feed ramps.