'04' is the Boeing company code for Britannia Airways. Just like '36' is the Boeing company code for British Airways.
So BA's 757-200s are 757-236s, and their 767-300s are 767-336s, etc...
The company code stays with the aircraft throughout its life so if you end up on Britannia's G-CDUO or G-CDUP then they are 757-236s because they are ex-BA aircraft (i.e. first delivered to BA).
Incidentally, Airbus don't do it that way at all. They use the last two digits to correspond to engine type, so for example an A320-200 with V2500-A1 engines is an A320-231 regardless who it was delivered to, and with CFM56-5A1 engines it's an A320-211 etc...