I recall that BSA offered different barrel lengths as a custom option, up to 30" in 303.

From what I've gleaned off of a number of forum boards, the No.4 Enfield barrel contour is very similar to that of the older Lee Metford/Lee Speed barrel (the No.1 barrel being thinner). However, the thread pitch of the No.4 shank is different than that of the No.1 by a few degrees (don't recall the details right now). My plan is to re-barrel one of my Lee Speeds with a Criterion No.4 barrel which hasn't been finally machined with the threads cut or the bayonet lug put in. I gave them a call (easy to do, as they're only about 10-ish miles down the road) and they thought it would be a pretty easy thing to offer.