Not quite true.
There is Zen3+ coming out later on AM4.
Please double check this, and link . Zen3+ for desktop doesn't appear on any roadmaps I've seen and AMD have made clear that Zen 4 is due in 2021 and is going to be DDR5, which obviously can't be AM4.
I've seen speculation from commentors that the chiplet architecture means they could in principle backport Zen4 to AM4 , but as far as I can see there doesn't seem to be a good commercial reason to do it.