Maybe 4.5, I'm not sure.
I am sure I really liked it a lot, though. Low angst, good characters, good romance. It left me smiling.
There were a few flaws; one plot element that seemed completely pointless and another plot twist that seemed just thrown in for the heck of it and didn't really seem to fit. But otherwise I liked it a lot. Compare to Heart Doctor by Drew Zachary - not just because they're doctors, but because the whole tone felt very similar.