First: Look at your bolt, does it have a spring inside it to keep the pin in place. The slot anywhere on your bolt is a dead give away
From the diagrams i've seen, it looks they threw the
"ACS" bolt, Anti-Chop, bolt in and called it a day. They did that with the MR2 and MR3 too and the same problem kept popping up. "why does my gun chop and when it does why do i have to keep re cocking it". Basically you need to break it in.
(easy mode). Get rubber bands and wrap from pin to bolt face. How ever many it takes to keep it compressed and leave it over night. Test the bolt, if you dont' like it, do it again.
edit: I can not find any pictures of the MR4 broken down. If your bored and want to post up a picture of your bolt that would be nice. I also couldn't find anything saying it didn't have the ACS bolt which was just as odd, but the problem you have has occurred before, but left kinda unsolved. Would be nice wrap it up once and for all ya know.