The community hopes to eventually move fully to Lean4. There's some details about the history of earlier Lean versions here, but they're not too relevant as the first two versions were very experimental. Now, some members of the community and Microsoft are diligently working on
mathport, which will hopefully provide a full syntax-ful translation of Lean3 code into idiomatic Lean4 code. The main test-case for this, at the moment, is
mathlib itself, and we are pretty determined to make it work.
This blogpost offers some further details on what to expect from the transition - there's still not been a consensus on how gradual the transition is, but I suspect that eventually there will be a day declared where there is an announcement "no more PRs to mathlib3 will be accepted, please move your work over to Lean 4".