Look for all the current A4 discussions. The A4 is a flat-top 20 inch with the KAC M5 RAS.
I have a set of the 20" YHM rails on my dissy. I like them. I know there are a couple of M4 builds on this site that use the YHM rails as well.
Oh, and on an HBAR, that thing is gonna be nose heavy with an M5 on it.