Saying Lionhead has confirmed something means nothing. Remember the acorn, people. Jack could show up in Fable 4, and they could say some evil mastermind gathered the remains of his masks, put them together, and then spent years of his life (Similar to how Lucien rebuilt the spire) reviving Jack's spirit, and put it on. Hell, they could even say that Reaver did it in an attempt to get out of his deal with the Shadow Court and stay immortal, not knowing he would lose his free will.
That being said, it likely isn't going to happen. Jack was a challenging boss, with depth and a clear goal who was a challenge to beat. Lionhead, while trying so hard to make Fable "accessible", are making their final bosses weaker than the first boss, which is sad. I killed Lucien with a rusty clockwork pistol that I found in the water of Bowerstone Market. I killed Walter while I ate a poptart and talked to my mom on the phone. If Jack DID come back, I'd want him to come back with the old combat system. Otherwise he'd have a final battle just as anti-climatic and lame as the Crawler and Lucien that it would leave a permanent stain on his legacy.