Lead time and time through all stages of manufacturing can take months though, so if they start manufacturing those by the end of the year they may not make retail until March 2022 or so.
Another unknown is how much this will add to the price. Certainly they will be more expensive to manufacturer and package but on the other hand 5000 series CPUs have decent margins; competitive reasons mighty mean that for x900 and x950 parts the price stays the same.
Zen4 DDR5 parts will eventually come too, but it looks like AMD are allowing Intel to launch consumer DDR5 first with Alder Lake.
Since initially DDR5 is likely to be on short supply, expensive and not really faster than DDR4 not hurrying to be first on DDR5 is probably a good idea.
Intel are anyways hedging their bets since Alder Lake will support both. AMD might do likewise and their CCD + IO chiplet approach would make that relatively easy.
No idea what it if Zen4 with DDR4 would look like. A final outing of AM4 would be nice, but it's probably wishful thinking.