Persona 4: The Ultimate in Mayonaka Arena will feature characters from both Persona 3 and Persona 4. Persona 4: The Golden will include a new character, voice acting, anime cutscenes and possible co-op.
Atlus is aiming for a January 2012 release in Japan for Persona 4: The Golden. Persona 4: The Ultimate in Mayonaka Arena will hit arcades in Japan next Spring, with PS3 and 360 versions releasing later that summer.
Arc System Works will be working on Persona 4: The Ultimate in Mayonaka Arena. For those who don't know, they are the guys who developed the Guilty Gear and BlazBlue titles.