Everything, really, since essentially everything has been updated. i.e.
submit every form with good and bad data
perform every action (including those in extensions)
check for anything out of the ordinary like blank white pages, or pages with errors
upgrade from version 3 to 4
create a new installation of v4
perform migrations from v2.5/whmcs to v4
check for obvious (or not so obvious) page styling issues
see if anything comes across confusing that you think can be improved
ensure current functionality still works
try out the Mass Mailer plugin
A dev system would be perfect, but where would you be importing data from? Not real data from a production environment, correct?
Sometimes people insist to me that they want to test changes in a dev environment where the data mirrors what is on the production server. It sounds like a simple and convenient way to test what is and what will be, for comparison. They'll just disable emails from being sent and disable payments from going through in order to be sure real customers do not receive emails or get charged from the dev environment while they are testing it. However, this is a horrible, horrible, horrible idea for numerous reasons. Not only does it limit what you can test (e.g. can't test sending emails or making payments), but bugs may exist that send emails, make payments, etc., even when disabled. All data in a test/dev environment must be test data. I hope that's the intent of everyone that will be trying out the beta.