At this time I do not own any M4s or AR/patrol type 5.56mm rifles, but I'd look at the new green laser designs from SIG Sauer or Crimsontrace.com.
Crimson Trace(CT) laser aiming units are very popular with spec ops, SWAT and military troops who are "down range"(SW Asia).
www.sigsauer.com www.crimsontrace.com