Blizzard will never create Warcraft 4 for the following reasons:
A) It conflicts with the potential story for WoW which is arguably Blizzard's biggest success ever. Why deter the people that are already playing your MMO with a 15$ subscription fee? How would you handle releasing Warcraft 4 among WoW's storyline? How would you release expansions for Warcraft 4? WoW will -NEVER- die to the point where Blizzard would want to stop releasing content for it in order to create Warcraft 4. Ultima Online still has a monthly fee despite it being ancient as far as MMO games go and they still release content for it. Knowing that imagine if WoW went free to play, it will -NEVER- die at that point.
B) It conflicts with Star Craft 2, another RTS that they are already producing expansions for. Why create 2 RTS games when Blizzard wants people to get into the SC2 competitive scene?
C) Also Blizzard said in the past that they have absolutely no plans on creating Warcraft 4.
We can only dream though.
Also this ^. Maybe you enjoy reading the books but the story delivered via the game is shit. (Blood elves and Draeni come from outer space!)