Try Ellwood Epps - on their website, 'This n That', then 'Misc Gun Parts.' About half way down they list Lee Enfield parts incl 'several' complete No 4 bolts. If they have a range of different boltheads, you might consider visiting them with the rifle to make sure you get the right fit. I bought a virtually unused LB 1943 serial no bolt off them a few months ago, a real beauty which now slides happily back and forth on my recently put-together 22 No 7 rifle.
Epps actually have in stock what they list on their website and apparently update it every couple of days or so, unlike some Canadian based parts suppliers on the web who have impressive looking lists of No 4 and even No 7 parts, but when you call them up there's nothing there ...
Another good bet is Springfield Sporters (you'll find them on the web), who are a very good supplier, but you have to contend with customs dues and now who knows what else with all the recent nonsense about US gun part import/export restrictions.