I can no longer edit the above, but ignore it all. There are numerous tables use the user_id and trying to update them all and get them in sync is far too complex. In short, if you want to retain membership numbers, DO NOT MIGRATE TO AMEMBER 4. If you have even one member record deleted in your old database, you're screwed.
Andy, I have removed your first message, as you agreed it had incorrect instructions which could mess database. There is a way to import users and keep membership ID, however it will not work if you already have users in aMember v4 database, so we didn't include it in aMember v4. Contact us in helpdesk if you are interested in such solution.