Sonic the Hedgehog 4: Episode 1 was released roughly a year and a half ago now, so it seems like it's been forever that Episode 2 has been in development, especially considering the fast rate at which games in episodic series tend to be pushed out. But fret not, because the next (and likely last) episode will see its release on May 15th on XBLA, and May 16th on PSN.
Episode 2 looks to include Tails along with Sonic this time around, along with Metal Sonic as one of the villains. The parallels to Sonic the Hedgehog 2 are definitely there, so it'll be interesting to see how much like that game it really is.
In addition to PSN and XBLA, SEGA is also planning to release Sonic 4: Episode 2 on iOS, PC (both in May), Android (in June), and Windows Phone (in July).