Not arguing for Mach 4 for J-36, but you don't need detonation engine for Mach 4. Ramjet works up to Mach 5 at least according to Chinese papers that I have read. Beyond that you need scramjet which are the detonation engines or rocket.
Just saying, but the J-36 certainly doesn't feature ramjets as well.
If they’re modeling both Mach 2 14 km and Mach 4 23 km that tells you the intended design range for separation and ignition. The modest read of these parameter choices is that they want to be on the safe side and cover a wider design range just in case. The more ambitious read is that this missile if it’s real is intended to be used by both Mach 2 and Mach 4 capable aircraft, and that a Mach 4 capable attack aircraft is in the pipelines.
14 kilometers = ~45000 feet.
I'd say that the posts on this missile would be more suitable for the PLAAF munitions thread, since (if the academic paper's authors didn't actually fumble with the values) this missile's discussion certainly goes way beyond the J-36.