It lacks one upgrade that came with -5 of a gusset that ran down the frame below the blast shield. And some say the frame mounted firing pin of the -5 is an upgrade, but ya get MIM with those 2.
Otherwise, the -4 is probably the latest of the greatest.
Damn nice find. I've got a -3 which is the last square but, but had I seen this for that money, I'd have it too.
