I know this thread is a little dead, but I have a side by side comparison showing the M4 is specifically limited in its HDR output - side by side comparison of 1440p HiDPi on the exact same monitor and cable of an M3 Pro and M4 Pro.
I've got a ticket in with Apple Support, but I'm 99% sure they've attributed my issues to the wrong ticket internally and that won't do us much good.
Still an issue on the 18.4 beta as well :/