Starcraft II will release with the same three races as the original games however Blizzard hinted that the fourth may appear in an expansion pack.
“We don’t have the resources or time to add a fourth race to the launch of StarCraft II, but I’m sure in the event that we decide to do an expansion set it’s a feature that’ll come up for discussion,”
– Blizzard Co-Founder, Frank Pearce
Pearce stated that the studio considered adding a fourth race to Starcraft II however decided to focus “the cool, best ideas on the existing three races rather than diluting those ideas across four races.”
Blizzard has yet to release any details on a release date for the PC, but the company is known for releasing expansion packs for its franchises.