Credit unions are a good choice, that is a competitive rate. Catholic Credit Union in my neck of the woods was running ads for that rate. Toyota isn't running any specials through Toyota Financial for the Prius. From what I can tell, they use some kind of index attached to some index, I'm guessing National Median Cost of Funds, but it could be literally anything else.
Anyway, I got 4.45%, which I thought was competitive for a non-program rate. I'm making principal payments anyway so that is also having a positive impact on shortening the term and reducing my interest exposure.