Neither of these are exclusive to high end boards, nor would running memory with Zen at 4000MT/s be wise in a lot of cases, due to the fact that getting the I/O die to run at 1:1 with 4000MT/s+ memory is an incredibly difficult task, and so running the I/O die in 2:1/ out of sync with the memory would only hurt performance, and as such, getting a lower frequency memory, i.e a 3600/3800 kit with tighter timings and having the ability to run it at 1:1 would be a better decision.
The main difference between B550 and X570 being the extra PCIE Gen 4 lanes provided by the chipset on X570 that B550 lacks, aside from that, and assuming you don't need the extra Gen 4 lanes, B550 could and are of the same quality as equivalent X570 boards.
What's your budget for a board, and what's your currency and location?