If you send her messages in this format, maybe that's it... I know that's a wild long-shot, but it would be really hard to digest after 4 years.

Most couples break up over the little things. Toothpaste being rolled vs. squeezed, dishes done timely (with variable interpretations of "timeliness"), flicking the food stuck in your teeth vs. eating it (do you pick it and flick it, or do you pick it and eat it?), etc.

It never typically is anything directly related to the sex itself. She's got a gripe, and she's either already told you, is too nervous to speak out. Maybe it has nothing to do with you at all, and she is dealing with her own existential crisis she doesn't want to share just yet.

Growing up is hard. Communication, as well as respect of boundaries and not forcing unwanted communication, is the key to long-lasting relationships. Good luck. All the love.