I’d keep the M4 — it’s battle tested/proven, unlike some of the other participants in the exercise.
In most environments except hard desert conditions you could not get me to give up mine. Parts availability and continuity are outstanding. Accessories, general knowledge, its all there.
However, I will not go below the full 16" barrel. That is to say 16 not counting the flash suppressor. M-4 barrels are around 14" and that is the VERY bottom of the performance curve for the 556 / 223. Velocity is right about 2,400. The 16" barrel will rock in at around 3,100 fps.
Your mileage may vary
...in that snazzy 6.5 x 284 NVDave is refering to I would give it some serious thought.