New launcher is listing 4.21 as “Preview 4”
Getting this as well.
I can confirm too, the engine seems to be the same, I think only the label is wrong.
On my MacBook Pro 13" mid 2014 is 4.21 Preview 4 not able to start. There is written “not supported”. I had to end the Beta in order to open 4.21.
We’ve shared this feedback with the Launcher team.