Some people have written that the magazines are interchangeable, but in my experience, and aside from the locking lugs on the back, there are enough dimensional differences that a No.4 magazine will have to be pounded into an SMLE Mk III, and then can't be removed. The British did not consider them interchangeable, and neither do I.
It is a good question why they did not design a common magazine but the fact is that by the time the No. 4 came along, the magazine was considered a part of the rifle and not to be removed. Spares were not issued.
The original British Lee rifles were issued with two fitted and tested magazines; the primary one was chained to the rifle so it wouldn't be lost. The other was carried in the soldier's pocket or "kit" and could be loaded as a spare in combat. But when the rifles were changed to charger (clip) loading, that system was done away with, and the single magazine was thereafter considered a part of the rifle.
Jim