The No4 MkI has a ten round magazine. The intent is the magazine remains in the rifle and the rifle is loaded with stripper clips. Some times you well see a sporter No4 with a five round magazine made to meet game laws. Aftermarket LE mags are basicly junk. The only successful aftermarket mag is the five rounder made by Golden State Arms, from the 1960's these are very well made and marked "Made in Japan".
The No1 and No4 mags get swapped around some times. While they well fit either rifle, the wrong mag well cause feeding problems. Easy to tell the difference.
The No4 mag the two stamped ribs stop short of the bottom of the mag and on the No1 it goes all the way to the bottom.
Before starting a restoration check head space, be sure it's a matched rifle as it is spendy to restore these. It may be cheaper to sell this one and buy another in correct condition.