What it's capable of - - -
The NRA booklet on the Enfield Rifles, mostly a compilation of articles from the American Rifleman magazine, is very worth having. I see it at gun shows, and you can order it from NRA Publicaitons Service. It shows some very interesting accuracy tips for the No. 4 rifle, including bedding information. Haven't tried it yet, myself, though.
An armory stock, decently shooting No. 4 is not necessarily the ne plus ultra in tight groups, but it IS surprisingly easy to shoot well. It is a mass-produced, really-rough-duty, battle rifle, after all, intended to hold together under adverse conditions, while roundly thrashing the enemies of the realm.
Make sure you're consistently on target, with the windage properly set, and then go as far back as you can manage. Set up some silhouette targets, or just sheets of cardboard, and give it a try. As mentioned above, a well-defined target at 400 yards is not beyond capabilities of the the rifle and milsurp ammo. It was conceived, not for sporting use, but to make adversaries cease and desist activities contrary to Britian's interests . . . .
Best,
Johnny