AMD is reportedly testing AM4 variants of Zen 4 internally. This rumor has been making rounds on the web for a while now. The same has been confirmed by @Greymon55, according to whom these chips do exist but whether or not they’re released remains to be seen. AMD has already stated that it’ll continue to support the AM4 platform, although in what way it’ll do so is unclear.
The launch of the Ryzen 7 5800X3D was the first step and the supposed 5600X3D/5900X3D will be the second. If AMD feels the pressure from Intel’s budget Raptor Lake offerings, then we may very well end up seeing budget Ryzen 7000 CPUs supporting the B550 and X570 boards. These chipsets come with support for high-speed DDR4 memory, going over 4000MT/s which may make them feasible to run the Zen 4 chips without much of a penalty.
Personally, I don’t believe that we’ll actually see Zen 4 on AM4. Lately, AMD has been focusing on the high-margin enthusiast segment, and releasing next-gen budget parts on an older platform just doesn’t make sense. Add to that the fact that the Ryzen 5 5500, 5600, and 5700X were launched just a while back. Team Red will be conceding the market to Intel in the low-end and budget segments, but that suits it just fine.