Oh, yes, absolutely. Everything's fine. You can use Firefox Sync. It works a charm.
This is because though the browser has changed, the data syncing is the same. All the history, cookies and stuff will be synchronized. Oh yes, be warned of this little thing though: The Persona on other computers will also be in sync, and the Persona will be applied to Firefox 4 too. But (though I notified Mozilla when Firefox 4 was a nightly build), when a Persona is applied to the new version, the Close, Minimize, Restore/Maximize buttons disappear, and you cannot close (or do the other things with) the browser normally. To make the buttons appear, you need to make the window smaller. The buttons disappear only when maximized.
Anyway, that's not a big problem and you can use all the stuff normally. If you're a perfectionist, disable syncing the Personas for the Firefox 4 computer (if the option is available).