In this interview with TV Guide Online, Sarah acknowledges that her fan support factored in the producers' decision to bring her back to the show.  The relevant bits:

How does it feel to be promoting the show again? Who made the first overture to get you back?
I'm not sure because they called my representatives. Apparently something had been kicking around for a little while. I was actually in the process of negotiating to be on another show, and this came out of nowhere.

Especially given the circumstances of Dr. Sara's exit.
Especially given that I had not come out a-head in the third season. [Winks] Let's get all those jokes out of the way. But no, it had to do with the fans. I'm enormously grateful for their persistence.