Yes, there are differences in the 4 position left selector. The protrusions or nubs that insert into the trigger housing are orientated just enough differently so that a 3 pos. or a semi selector will not insert..Also the slots in the left side of the trigger housing are facing toward the rear of the grip. I have a 0-1-2 selector but it will not function in all 4 positions. One of those 0-1-2 selectors could be modified to work ,it appears. I believe Aviator Dave had posted pics of a 4 position selector but that was some time ago.