The BUIS concept was designed for just that purpose, to provide a backup to use with the standard M4 front sight assembly when an optic is used as the primary.
Im unaware of any rear that doesn't align, the ones I have experience with change the front plane (eg a gas block installation), but they still align correctly.
But that is just my limited experience with them (MI, Troy, YHM). Some of our more well rounded guys may have better data.
"I'm your brother, just make it count."