Synthetic stocks for the No.4 (also fit No.5) rifle are made by Ramline and by ATI. Both are claimed by their makers to be no-gunsmithing-required, drop in fit. Both brands get good and bad reports from users.
The ATI has a monte carlo style butt-stock so it's probably better if you want to use a scope, especially as most scope mounts for the No.4 and No. 5 rifle put the scope higher than on most rifles. Because of this, if I were buying another I'd get an ATI again even though the one I have is defective (got it secondhand with a sportered rifle that came with the wooden stock, too, so no warranty and I can't be bothered to complain, I think the seller didn't figure out what was wrong with it, I had to get a gunsmith to work it out for me.)