AFAIK, the only 5nm parts on AMD's leaked 2021 roadmap was Genoa/EPYC, mainstream stays on Zen 3 through the year.
With AMD having a 14-15 months delay between Ryzen launches so far and Zen 3 unlikely to have broad availability before 2021 due to the announcement coming so late in the year already, AMD will probably end up announcing mainstream Zen 4 about in time for the xmas/new-year break.
The Zen 3 announcement is tomorrow, so we'll see soon enough.
Intel has released practically back-to-back generations before with desktop Broadwell which launched in January-June 2015 depending on model getting replaced by Skylake in August-September 2015.
When Intel wants to push and isn't running into a process brick wall, it can move quick.