A mag for a #4 doesn't work in a #1mk3, so unless there is a recognised conversion of #4s to semi-auto/full auto, a modern made mag for a #4 wouldn't need to be exempt; it already is.
I have seen M-14 full capacity mags in use on an Enfield .308. The guy who did the modification welded a stop on the mag housing so it won't fit into a M14.
Sometimes I think we are our own worst enemies. Bolt actions are exempt from mag capacity; there are no semi-auto Enfields in Canada, and yet we are debating about the possibility of prosecution for an above 10 round magazine.