Hi Frank
I make a single stack insert which fits in a No1 Mk3 SMLE magazine,to take the 375-2 1/2 shell, I realize you are after a conversion for a number 4, but I am sure you could adapt my design to suit,
I still haven't mastered posting pictures on Nitro, but if you are interested send me your email address and I will send you some photos.
cheers
Darryl