It’s a free upgrade, and all 60,000+ user accounts will be migrated one at a time (really, really fast). It takes a few seconds per account to be migrated, so this should go on through most of Sunday, and we’ll be live again on Monday morning (pls. note the emphasis on "should").
Since there’s no telling exactly when your MailChimp account will be migrated to v4 on Sunday, we recommend waiting till Monday before you log in to make any new content. Pre-existing scheduled campaigns and tracking should be fine (unless you scheduled a campaign to go at the exact same moment your account was being migrated). Regardless, any rare cases of un-sent campaigns can be re-sent on Monday morning.
We’ll be posting screenshots and details and tutorials here on the blog starting Monday morning. You can also join the discussion over at The MailChimp Jungle.