It might be. I would think, in all honesty, that in four years of sexual activity together that if there were anything to worry about (barring HIV, which if you tested negative for already, you're fine), it would've manifested itself already. Granted, that is me speaking off my gut with no real medical authority, but I think you're well past the point of worrying.

Were I you, I'd focus on simply being there for her...she revealed this to you out of trust for you. She felt it was something you should know, and her being able to trust someone else with the information is likely a huge source of comfort for her in and of itself. Couples counseling might be beneficial as well, as it would allow you two to talk about this matter (and others) in a healthy environment.

I wish you both the best as you work to heal together.

Hugs,
Harley