I think there would quite a lot of takers for an M4 Pro with 36GB, which is enough to satisfy many users who need more RAM than CPU power and who don't want to be forced to wait and pay more a Mac Studio. I would guess that most users who want 48GB RAM also want more CPU and GPU cores and would be looking at the Studio anyway.
Starting at 24GB RAM would hopefully mean only a $200 increase to upgrade to 36GB, whereas the M2 Pro required $400 to go from 16GB -> 32GB.
I hope Apple doesn't create the same artificial boundaries as per the M3 Max which has a massive jump from 36GB (for the binned SoC) to 96GB for an extra $800. People who buy a binned M3 Max probably want options for 36GB, 48GB, and 64GB. I am assuming that the binned M3 Max uses only 3 of the 4 RAM modules which is why we get 36GB (3 x 12GB) and 96GB (3 x 32GB). I could understand why there is no 64GB (not easily divisible by 3), but I don't see why they didn't offer a 48GB (3 x 16GB). The only reason is to push buyers to the unbinned M3 Max and have to pay an extra $500, even if all they wanted is an extra 12GB of RAM for $200.
I shall not be happy if the M4 Pro options are 24GB or 48GB for an extra $400.