There were three or more slings in use militarily. Some were Brit and some American. The Brit sling was a very thick and robust strap with high beaded edges that gripped a brass buckle comprised of hook like claws that held to the beaded edges of the thick strap. The Brit sling was normally khaki colored but came in OD Green, white, and dark blue. There was a leather sling that was Brit intended for earlier Enfield's but sometimes issued for the Mark 4. The Americans built many of the buckle free Kerr Slings for many different arms to include many Enfield variants and even the Thompson submachine Gun. Some Kerr slings were tailored exactly for the Mark 4 and original Kerr slings can be pricey.
I have a Mark 4 and it is in original condition and I hunt with it and bench shoot it. It is a very rugged and versatile rifle and .303 will drop a Moose or a rabbit. Buy the Rifle. Buy some dies. Or buy a small Lee hand loader kit. You will be just fine with this rifle and never cut it or modify it. Never hot load your ammo as the round has enough power in normal loadings for hunting nor shooting. Clean and oil it often and give it to your Grandson to cherish when you grow old. That is what Enfield Rifles are truly for.