There's no way for me, or any attorney here on avvo.com, to predict what the outcome of your case is. What might happen depends on several factors: Your record, the particular facts of your case, the court, the jurisdiction, the judge and the DA. Only an attorney who has access to the police reports and can discuss the case in a face-to-face consultation would be in a position to give you a guess about possible consequences.
With the circumstances of him accused of a horrific crime, the DA may not wish to pursue this, as the "victim" would be subject to cross-examination about the rape and he'd decline to testify, pleading the Fifth.
Talk to a local criminal defense attorney. Most importantly - keep quiet about this. Don't speak to anyone except an attorney.
If you have no criminal record and the only charge against you is 4th degree assault, it is unlikely that a Jefferson District Judge would send you to jail. You may well get some SUSPENDED (or conditionally discharged) time, but no time to serve.
You may have to pay a fine and costs. There is a good change you will be ordered to Anger Management Counseling.
While there are no absolute guaranties, I would not be too worried about actually going to jail over this. Of course, I only have access to the information presented here.