Due to the wonders of eBay and thousands of yank shooters making project ".308" Enfields, the mags have become very scarce and expensive.
A genuine Enfield mag will go for £90-200. A Charnwood repro about the same cos the photo looks the same on eBay...). A Stirling mag will be cheaper - but of course does not include an ejector tab like the Enfield. An Ishapore 2A1 mag intended for a No1 action will actually fit most No4 actions. These used to be available by mail order from the US suppliers such as Numrich ( Numrich Gun Parts Corporation ), but they might have a stupid export ban in place by now.
(a) ebay (280998122435).
(b) next two weekends there are arms fairs at Bisley. There will be one or two mags - at a price - and usually Fultons can find one - at a price. Try Terry Abrams, whose stall is usually behind the bar in the main room of the arms fair.
Charnwood bought the tooling from Enfield (as well as all the remaining Enforcer parts, etc), which is why they made a load of "repro" shells to use up the genuine followers and springs they had. For some reason the Charnwood magazines usually don't fit, even though they appear identical to the Enfield ones. The location of the ejector tab and the height of the rear lips is of critical tolerance, even though the mag itself can be rough tolerance.
I am sure the T4 was only supplied with a 303 Mag, under NRA rules as a target rifle it was single shot only so the magazine was just there as a loading platform. The L42/L39 mag will only fit and feed rounds if the underside of the action in the Mag well has been machined out.
As said the Bisley Arms Fair this weekend, but expect no change from £100 and no guarentee it will feed rounds if the action is not altered.