What I do with mine (14").
-Light (what light depends on my application) on IWC. PID is all.
-T2 on a Scalarworks. A dot improves precision, speed, and allows more complete visualization of the target than beads or irons.
-TTI safety, black. This safety allows you to go hot by depressing it using the middle ("belly") of your finger either on the way, or into the trigger guard. It's an excellent way to keep the gun safed, finger off the trigger, without compromising time to shoot if necessary. A must have, imo.
-Lead faucet tactical sling. A sling is to a long gun what a holster is to a pistol. Admittedly, I remove it for my shotgun as its a snag hazard and mine is mainly used for coyotes near my coop.
-OEM aluminum trigger guard. No need, I just like it. Nod to tradition.
-Bonus...I replace the LPA rear ghost aperture with an LPA made aperture that lacks wings/dots. Its less visually cluttering and the way the dots line up (mickey mouse) really messes with you if you're used to literally any other dot containing sighting system.
These mods are not fancy, but they are things I've learned over nearly a decade and a half and owning around 10 m4s, that actually enhance value and function and make it a better weapon for my uses. The trigger guard is a nod to tradition, I admit, though.
Also...take it down, wipe it off very well, this includes the ID bore of the recoil extension. Then lube generously. I have come to strongly prefer cherrybalmz black rifle balm, or better yet, the forward design control version of it. Lube carrier rails, carrier bore, cam pin track, and bolt lugs, as well as the recoil spring and tube. Also, the tang of the barrel. A thick grease works well here if you like. Damps vibration a tad. Not necessarily, but will keep interior of receiver ano a hair nicer a hair longer.
I didnt shorten my list to fit the OP. This is my list of stuff to get an m4 sbs up to speed, for me.