I have only used transporter to move from 4.1 to 5.0, so I can't comment on how long the 4.0 to 5.0 move would take. I guess it is a matter of testing with a test user.
Would it be easier just to reactivate everyone on the besx server rather than move them?
That depends on what kind of user you have. We have lots of travelling users and many don't want to be bothered with re-activation, so a transparent upgrade without user intervention was much easier for us. But if you have all the users inhouse and don't bother helping them with re-activation, you would not need the transporter.