Well then, it's clear that H and N need to reconnect and get back to loving and romancing each other. They should be having hot dates together, just as you should be having hot dates with M.
I think it can only help the entire quad if the pre-existing couples step back from their OSOs, strengthen what they have together, and find the good stuff again, before going back into "swapping" positions. Then, take that slowly! Don't let NRE swallow anyone up to the point of ignoring someone else. Make an agreement that all of you speak up when you see that happening. Perhaps you should also all go out and socialize together as a foursome on a regular basis as well.