There is no M3 mini so can't compare (the entry level M3 MBP was able to drive a 4K display up to 144Hz but with the usual 3072px horizontal HiDPI limit which translates to a 6K horizontal framebuffer width limit). The M4 can also output 4K@120Hz (can do it up to 240Hz actually) just fine, but HiDPI resolution still seems to be limited at 3072px horizontally (for all refresh rates) for 4K displays for some reason. Obviously this is not the case when I connect a 8K display over HDMI (like a Samsung 8K TV) - in this case the mini has no issues going up to 3840px HiDPI horizontally, since this is needed for a proper 8K output. Also, the horizontal GPU limit according to the ioreg framebuffer entries is 3840px horizontally which is an obvious increase from the previous 3072px for entry level macs, so the limit in display mode list is clearly artificial. Maybe this will be updated in 15.2 - I am yet to install the latest beta on the m4 mini.