I think there is an error in reporting. Firstly, a woman cannot be charged with rape unless she aids a man in committing it, not the case here. Secondly, if she was charged with rape, the bare minimum is a 4 year custodial sentence.
The more likely scenario is that she was charged with 'sexual activity without consent' which explains the more lenient sentencing.
Just realised that she was given a 4 year sentence, I got confused with the time she has to serve in prison.