My team plays a lot of night games, and under those circumstances I can tell you that WITHOUT a RD sight, you can be at a huge disadvantage. During the day, however, it really just comes down to personal preference. I've used both, and I just prefer a RD still, day or night. One thing I will suggest for an M4: make sure that you're front sight isn't blocking your line of sight. Usually this means mounting the optic on top of the carry handle, or removing the front sight. Hope that helps.
-BATMAN