Yes, that mysterious cloaked figure in Fantastic Four is indeed played by Robert Downey Jr. – as confirmed by the director himself.
In the first of The Fantastic Four: First Steps post-credits scenes, Sue Storm stumbles upon Doctor Doom in the Baxter Building, taking an interest in young Franklin. Just what the supervillain wants isn't clear yet, but it most likely has something to do with the Power Cosmic residing within the kiddo.
