Serial number question No4Mk1

Gents, a friend of mine has just bought his first Lee Enfield. He asked me who the maker was.

It is apparently 1943 date; but the serial number has only four numbers after the prefix and the first one is a 4 It is AP4***.
I know the basics of Maltby starts with 1, Faz' 2 and BSA 3; I was wondering if it could be an early production BSA number because of the four digit number; but I'm really not sure.
In the photo I've seen it's definitely a No4Mk1 without the *

So over to you Gent's to help me out on this one!
Thanks, 30Three.

It's spot on for a 1943 BSA. They didn't start using the five figure serial (preceded by a letter in all but the first block) until the beginning of 1944. During 1943 they seem to have used two letters, the first of which was 'A' & the second of which proceeded through the alphabet; AA, AB. AC. AD etc., followed by four digits. Thus, AP 4XXX would be a mid (ish) 43 rifle.

Thanks Roger; That's great.