You should’ve done it the other way around indeed… Try Whisperfish and then upgrade 
Currently, no, there’s not even a workaround. If you have an existing 3.x installation, you can register there and copy the data directories.
While this is indeed a “high priority” bug, none of us have a clue how to fix it at this stage. We have forwarded our query to Jolla. It seems to be a breaking change somewhere in the new 60 ESR release that they introduced (and rightfully introduced!)
While this is obviously a blocker for any “release candidate”, I would rather await Jolla’s feedback at this point. @gamag and I both have spent numerous hours working around several issues to get captcha’s to do anything at all on 3.x, only to be faced two days later with 4.x breaking our work. These are countless hours that I would rather spend now on getting UUIDs to work correctly, because these actually cause our current user base to lose communication with several people (i.e., new users and GroupV2).
That said, a implementing a workaround to get 4.0 users registered should be doable, but it will involve using a second computer and using the browser console to capture the captcha token from Signal.