There are rumors suggesting the replacement to the EOS-5D Mk III will not be called the EOS-5D Mk IV. Instead it may split into 2 branches, EOS-5D X and EOS-5D Xs. The former designation will be the straight forward successor to the Mk III while the latter may be a version geared towards video users, replete with 4K.
This rumor is not new and the move is not unprecedented. Canon already split the EOS-1D model into the EOS-1D X and EOS-1D C. Frankly, I don't see the need for such a move. The splitting done on the EOS-1D model is not exactly a ringing success. In fact, the EOS-1D C had been heavily discounted and will be subsequently discontinued before long. I rate this rumor with about 30% chance of probability at this time. You can read my post on the upcoming EOS-5D Mk IV here.