If the fore end is tight, just leave it alone. I wouldn't spend any more money on it, other than to have it drilled and tapped for a scope mount then put a scope on it. The side mount pictured, is a very good mount and tough as hell. Many mounts for No4s are mickey mouse and to light.The 10 round mag doesn't hurt anything IMHO but if you really want a 5 rounder, just trade it for one. They run about the same price and it should be a straight across trade.
Many curl their noses up at such rifles. IMHO, someone liked that rifle well enough to spend a lot of time on it. It is probably a decent shooter as well. If the fore end has dried out and shrunk, you may want to glass bed the ways and other contact points.
Shoot it and enjoy it. There are many here that would love to have such a rifle. Light, handy and accurate. You can't ask for much more.