That is a pretty good find. There were around 20,000 of them made, and so they are pretty uncommon. They did lead to the Mk VI, which was the progenitor of the No 4.
It is a bit hard to find details of service in WWII, but given the desperate shortage of rifles in the early part of the war, especially following Dunkirk, I'd be a bit surprised if they weren't pressed into service somewhere.