No, they did not produce a No5 on a No4 action - BUT - during the Malaya crisis the Armourers would replace a No5 damaged action with a No4 action rather than wait for weeks for a No5 action to arrive.
HOWEVER - it would not have left the armourers shop shouting loudly "look at me, I'm a No4 Mk1", at the very least the markings would be barred-out.
Several US importers 'improved' their sales of slow moving NO4 inventory, by manufacturing 'tanker rifles', it could be one of theirs.