WC4 thread pops up from time to time. I would say the chance WC4 will happen is almost zero because
1. RTS market these day is not large enough compare to MMO,MOBA,FPS or any casual games out there. It doesn't justify the cost of making epic RTS with a lot of cinematics and high production value.
2. SC2 sold around 6 million copies not including HotS so now it's fastest selling RTS game of all time but you know what? the sell pales in comparision to WoW or D3. HS will likely give more money to Blizzard in the long run too.
3. RTS genre in general simply doesn't suit the direction gaming market is going these day. The industry is leaning toward easy to pick up and mechanically accessible games. RTS genre hardly has them.
4. RTS market has transformed itself into MOBA market. MOBA is the form of RTS that has been "changed" to cater to players these day.
All of these are coming from a big fan of Blizzard's RTS games especially StarCraft series like me. My prediction is that Blizzard will likely not make another RTS game after they are done with SC2. They will shift their gear to Blizzard Allstar.