I just finished upgrading our "backup" membership site from v3 to v4. Instruction were simple and the products and user migration went smooth; however, the newsletter migration failed (not sure why). Nevertheless, if you are TESTING the upgrade process on a development box (with your v3 membership data) before going live with v4, turn off email first. Otherwise all the members from the import will receive a confirmation email when you "rebuild db" at the end of the migration process. I had to send out a "Disregard Last Email" to all my members after upgrading the test server. It wasn't big deal, but thought I would post the experience as an FYI.
It is a serious problem, thank you for reporting! E-Mails are disabled in the import process, but we did not think about rebuilding. It will be fixed.