Lockdown is not to prevent spread, it’s to protect the NHS and the vulnerable - preventing spread is just a means to that end. I honestly can’t see any scenario where the public will in any amount tolerate and obey a fourth lockdown. Nor will they lockdown if people aren’t being hospitalised anyway, at some point this is just going to become something we deal with like the flu, whether we like it or not eradication isn’t an option.
Besides: Unless we totally mess up the vaccine distribution how exactly is a scenario where we need to lockdown again meant to occur when deaths and cases have been in free-fall for months now? Unless it turns out one of the strains just outright bypasses the protections provided by the vaccine of course, which hopefully won’t happen.