I'd be surprised if he did Indy 4. For one, once you take the fanboy element out, the issues that Indy 4 has are, generally, the same issues that can be leveled at the other Indy flicks. Also, I wouldn't be surprised if the RLM team is burnt out on Lucas for the moment and save an Indy 4 review for another time.
Personally, if they are to do future reviews, I'd like to see them take on something like The Dark Knight or Inception. I realize those are well-received movies, but it would bring more attention RLM (like the Episode I review did), and the reviews don't have to be negative reviews (like his Avatar and Star Trek (2009) review).