This isn't meant to be any kind of suggestion or request for TF4. It's pure, 100% fanboy speculation.
Supposing for a moment that Shia Labeouf/Sam Witwicky isn't coming back for Part 4. Should Bumblebee leave with him? This could actually make sense, seeing as how Bumblebee has been filling the role of Sam's guardian since the first movie, and he could simply be holding down his job by accompanying Sam to Britain with Carly (or wherever the screenwriters decide to send him). Would this allow room for a new, youthful Autobot protagonist in Bumblebee's stead, i.e. Hot Rod?
(Another alternative to Hot Rod could be Cheetor, if they're worried about recycling the plot of the animated movie.)