Out the gate, recoil,extractor, and hammer spring should have been changed to new at 4K.  With them weak from wear, allowing the bolt to cycle faster.  Also, take a look at the extractor claw, and if it has dulled, replace the extractor as well.
Note, if you do replace the extractor, make sure to de-bur the ends of the grabbing claw edges, as well as the end of the rim relief channel.  This way, the new extractor can get a full clean bite on the case rim during the pull.  As for extractor spring, tend to favor the Gold Colt M-4 extra tension springs, since they don't need to have a #60 O ring added to them to increase the tension of the spring on a carbine rig, and they will last the 4K before replaces without going weak in a few K rounds.


Next, with new parts in rifle, full charging handle pull, and make sure that the front face of the bolt, stops 1/4"~1/8" in front of back edge of ejection port window.  Over time if the old bolt stop distance was on the 1/8" side to start with, then wear to the buffer bumper will have the bolt face stopping at back edge of ejection window during hand pull/farther back during actual live cycle, and now spent case will be tagging back of ejection port as the spent case if coming off the bolt at back of stall (enough to defect the spent case back into the action).

Last one is a quick check of the ejector to make sure it still moving cleanly in the bolt face channel, so hook a spent case on the extractor, and use the spent case to cam it in a few times/allow it to pop back out on its own.

At this point with new parts, and rifle cleaned (including chamber brush scrubbed) and upper receiver area relubed with Fresh CLP, go load a single round into the mag, charge it with empty mag still in the well, and make sure that the bolt is locking back on the bolt catch, and the spent case is ejected 10'~15' out the ejection port.  If the case goes less distances, dribbles out the ejection port, then time to figure out if the rifle either does not have enough gas, or too much instead.

Simply, pick up the spent case and look at the rim of it.
Rim bent backwards from the extractor, too much gas and will need to either go to a heaver buffer to slow the bolt unlock down, or maybe to an adjustable gas block or key, from the plasma wear to the barrel gas port that is allowing too much gas.

Rim not bent, then either binding or gas leak instead.  On gas leak, tube to key and gas block, gas block to barrel, or key to top of carrier.  At 5K, gas rings could be suspect, but if you can pull the bolt up, and it stay up in the carrier, gas rings are fine.
As for binding, pull the bolt off the carrier, use just the carrier in the upper, and make sure that the gas tube is entering the key cleanly. If tube needs to be tweaked, do so over the center of the barrel, not back inside the upper receiver to bend the tube there.

So bottom line, rifle just needs to be checked out every 4K with a servicing, to replace worn springs and other wearable parts that last about that time frame as well, to get it back  to working spec.