As we all know Super Saiyan 4 does not appear until Dragon Ball GT. However, SaiyanIsland and Crunchyroll have found evidence that could hint that the true form of a Saiyan beyond Super Saiyan 3 is a "Super Saiyan God." Here is an article from Saiyan Island:
New scans of Dragon Ball Z: Battle of Gods have been released! The scan with Goku and Bils clashing (which is is the standard promotional image for the movie) teases something beyond a Super Saiyan 3 – a Super Saiyan God!
What exactly is a Super Saiyan God? A legend among legends. The Saiyans are the strongest warriors in the universe and the individual who holds a certain legend is the one known as Super Saiyan God. The scan mentions the form and the power of the Super Saiyan God will be revealed in the movie.
The scan with the Prophecy Fish mentions 39 years ago it foretold the appearance of the Saiyan God to Bils. It’s one of the reasons that led to Bils awakening from his slumber when he did.
Just as a note: Super Saiyan 4 was part of Dragon Ball GT, which was not done by the original creator of the Dragon Ball series (Akira Toriyama). The true level above Super Saiyan 3 is a Super Saiyan God.
If you look at this soundtrack to the movie you will also notice that tracks No. 31 and 32 are titled "Super Saiyan God is Born" and "Super Saiyan God’s Power." Curiously enough the tracks come after another track called "Super Saiyan 3's power," which is the 10th track in the list. If this is true it might mean that GT was never canon, if you ever thought it was canon in the first place. At the same time, it might not mean anything at all and it could just be a fusion or something. We won't really know until the movie is released, but that does not mean we can't speculate!