The threaded SS pin that is used for the bolt catch is not 4-40. Only the takedown pin setup is 4-40.
You can do this (for the takedown pin modification) on any AR lower receiver, small or large frame, doesn't matter if the takedown setup goes in from the back, or from the grip area (only on some .308ARs). I use 6-32 as well for this, because 4-40 set screws aren't always easier to find in local hardware stores, but the 6-32 are everywhere. There's a write-up that's about 10 years old on this mod, somewhere on this site. If you have the 4-40 set screws, you just run a tap in the hole that's already there. If you can't find 4-40s, and you do the 6-32 setup, you need to run a hand-turned drill bit through it first, before you cut the 6-32 threads. On some (all AR15s), you have to cut down the spring length. On .308ARs that go in from the bottom, you have to cut down the spring length. On .308ARs that go in from the back (like an AR15), you need to come up with a spacer, and you DO NOT cut down the spring.