So the AM4 socket backwards compatibility is partially backwards.
I have never ever seen anything say that backwards compatibility would be guaranteed just that the socket would remain used, I expect the same as last time at best, possibly partial support or no PB3 or whatever or pciex4 ,at best.
With x370 your probably already at three generations of CPU to choose from , with compromise.
I think if you bought Ryzen 4000 and put it in anything less than an X570 your wasting potential anyway personally, I wouldn't consider a three year old board a viable home for what will be a comparatively higher valued processor.
Sorry if that opinion hurts , it is genuine and personal not malicious.
And those arguing pciex4 doesn't at all matter are going to be singing a different tune in a year IMHO, the ps5 will push that trend, probably the Xbox too.