The Repeater field (and all ACF Addons) are now available as separate plugins instead of activation codes! You should be able to find a download link for the plugins you’ve purchased by logging in to your store account at http://www.advancedcustomfields.com/store/account/ and poking around through your previous purchases.
The upgrade path from v3 to v4 is fairly straightforward – update your core ACF through the usual WP plugin update routine, then install and activate your new ACF addon plugins. The Repeater field should show up exactly as you had it before 🙂
