I'm not so sure about RDNA 4 being out of the table, remember ever since Sony switched to AMD they used the latest and greatest GPU architecture available on the year of release on PS4, PS4 Pro, PS5. So they clearly have early access to the architecture to make these decisions. Though sure they might skip some features here and there that arent needed for the console just like Cerny implied they did for PS5 with RDNA2.
Zen 5 yeah... Judging by Cernys past comments i rather be pessimistic and assume Zen 2 and be pleasently surprised down the line with something like Zen 4. I remember him saying they did not upgrade the PS4 Pro GPU because it wasn't a new gen and made similar comments about things like CPU, SSD defining new gen hardware.

I would hope 4 years from now PS6 will use at least Zen 6, Zen 5 would be very outdated by then