I haven't seen any mention of an S4 either way, but going by the general reception of the S3, I doubt we'll see a successor at the same level.
I say this because there have been basically zero third-party contributions to Tizen OS which leaves the platform severely limited still 18 months later. There is no native app for messaging such as Messenger, Hangouts, or WhatsApp. Most fitness oriented users rate the fitness functions as mediocre at best, with devices costing half or a third that of the S3 far outperforming it.
While I do believe it is an amazing piece of hardware, it's really just a glorified notification accessory with its lack of software support. For $300, one would expect the watch to be able to operate much more independently, at least when paired with a Samsung phone. But if it is not a text or phone call, I have constantly pull out my phone to do anything. Really, my Pebble Time was much more capable when it came to messaging as it would at least store messages from any app on the phone for retrieval and reply.
Honestly, if the wasn't capable of looking a real watch in design and watch faces, I probably would have replaced it long ago.