It's a sensitive situation and largely depends on the kind of relationship you have with your therapist. It does remind me of a similar situation I had with my T - I accidentally found out his birthday online (I saw it when he started adding some of our mutual friends on his facebook list). When the week of his birthday came, I did say happy birthday and even gave him a gift - though I admit I was very nervous about it, not knowing how he'd react to my knowing information he hadn't shared about himself. He was actually quite pleased. But then, this is one particular situation. If you've known your T for 4 years, surely you can take a guess as to how she might react?