I'd say that Blizzard is already working on WarCraft IV, mind, in very early, "draft" stage.
Considering time periods between major releases and the leaked plan (that, though has not been confirmed, has been proven to be quite accurate by recent releases), I'd say they'll focus on WC4 after the next WoW release.
Another thing we might consider is the fact that the WoW plot needs only one thing now - and it's a nice RTS-style story telling. The days of the old foes, who were considerably strong, yet weak enough for a party of nameless heroes to defeat, are gone. Now we have no real enemy, a lot of new races introduced, and the focus in the story falls on the political affairs and warfare.
They will be able to milk two more expansions (heck, I won't be surprised if they actually make just one +10-levels expansion, but still I believe that there's plenty of milk in WoW - and, no matter how you feel about this fact, it's just business, so they won't be willing to waste resources). However, I do feel that WC4 is inevitable. The only thing that might stop them from working upon WC4 is SC2 mapping - which, for now, is fairly weak and too complex for the average Joe. Still, even in this situation, I won't be surprised if WC4 and SC2 are actually integrated within each other in terms of map making. That means that WarCraft 4 would be a separate game, but the world editors would share common features and even resources between games. It's a win-win situation both for Blizzard, who will be able to milk both SC2 and WC4 because of modding functionality, and for map-makers, who will have a bigger resources base.
Speaking of story, I am one of those people, who does not believe that we'll be dealing with our major enemies - the Burning Legion or the Old Gods (especially the Legion) in WoW. We won't put an end to them simply because they are an already working formula, and their destruction would mean that Blizzard will need to create something new - which won't be proven to work. Given the vast numbers of Warcraft players that WoW has created, my guess is that Blizzard won't be willing to risk and will actually continue the WoW story IN WarCraft IV, to attract MMORPG players who haven't played the Warcraft RTS games.
My guess for future expansion is that we'll have 4 races again, two of which would be the Humans (or Alliance) and the Orcs (or Horde, though I don't believe that Forsaken would still be part of it), and for the other my guess would be the Legion and the Old Gods (Naga? Nerubian? I won't dare to guess, actually). The good old RTS + RPG gameplay will return, both because it will be somewhat familiar to WoW players and because Blizzard rarely goes off the road by making major gameplay changes (WoW being the only notable exception). As for the setting, my guess would be Azeroth again, because of similar reasons.
Now that I've answered this question, I have to admit that I'm quite interested whether or not we'll see a sequel to the Lost Vikings series. Now that's something that we can dream of!