Answer 1

Go with Bootstrap 4! Even I think that Bootstrap 5 is also there but I prefer to go with Bootstrap 4 ( reason: better than bootstrap 3 )

Answer 2

Bootstrap 5 is the future, you should start to learn and use it for your new project now.

One biggest breakthrough of Bootstrap 5 is it's no longer using jQuery codes in its library, that mean its codes run in more efficient way now, and much easier for other JavaScript frameworks to do integration.