I have 5 female rats all living happily together.

I know that they come into season for around 12 hours every 4-5 days and when they are in season their cage mates may hump them, well one of them has been humped for 4 days in a row now.

I was wondering why this could be? Is she still in season?

I don't think they have to be in heat when they mount other rats. They do that to show dominance. I wouldnt worry about it. Its normal.