With World of Warcraft's next expansion Legion having been announced, it would certainly sound like as far as the Warcraft franchise is concerned, Blizzard is still focusing their efforts on World of Warcraft as opposed to working on Warcraft 4.
However if you were a fan of the Warcraft RTS, here's some good news for you. The folks at IGN managed to speak to Blizzard during Gamescom and during the interview, it was revealed that the developer could be considering the next Warcraft RTS, but the catch is that this will only happen once StarCraft 2 is done. This was confirmed by StarCraft 2's producer Tim Morten. According to Morten, "It's very rewarding for us to hear that there's demand out there for more RTS content in the Warcraft universe so once we're done with Void I think we'll get together as a team and talk about what would inspire us to work on next." However this is not a guarantee, but rather the team being open to discussing the possibility of a new Warcraft RTS.
In fact according to Morten, it is possible that they might just end up working on a new StarCraft game instead. "There's no question, though, that we'll consider Warcraft, StarCraft, or even new ideas. Anything is possible." StarCraft 2: Legacy of the Void will be the latest chapter of the StarCraft 2 series. It will also be the last chapter and will close the book on the current storyline.
This means that if a new StarCraft game were to be made, you can expect to see new characters. That being said it will be interesting to see how a new Warcraft RTS will fit into the World of Warcraft story. Given the latter is an MMORPG with evolving stories and lore, we have to wonder if a new Warcraft game could lead to potential clashes.