Here is what spoilerfix.com says about it:

03/27 - Network sources tell me that the voices of you, the fans, have been heard. They are bringing back our beloved Dr. Sara Tancredi, Sarah Wayne Callies, as a regular for season four! Prison producer Matt Olmstead [revealed that if Sara does come back, she and Michael] "are finally together, but like I said there have always been these physical barriers between them. He was incarcerated, she worked in the prison hospital, so obviously there's a barrier. He was on the run, she was in another part of the country, there's a barrier. Here they are finally able to be together, but we are jumping ahead in time, so there's that period that's unaccounted for, so what happened to her, what happened to him and hope they both help each other get past that, get past these. invisible barriers. There's no physical barriers anymore between them, but they're both pretty banged up when they see each other, and it's about how they get past that to really find that real intimacy. [...] They're all still trying to clear their names avoid jail time avoid death and so there's still that locomotive feel to it, storytelling-wise, and while that's going on on, on the fly, these two people are trying to save each other." Source: Kristin on E!Online