Assuming by "M4 type" you are referring to the A3 Flattop; In most cases they will be small pin, whereas you will need an "offset" adaptor pivot pin for replacement. They are available from Brownells, DPMS, Bushy, and so on. Someone on the board probably has one also. If you have problem finding one, email me.. I'm sure I have an extra laying around here.
I also recall someone making a run of large pin flattops.. can't recall if it was Les Baer or other (members - Help???)
As a note, the offset adaptor pin also screws from both sides. There may be more options available.. see what other members suggest also.