You could just ask them directly (but tactfully). Say something the lines of "So I'll have to be honest, I was very surprised to hear there would be a fourth round interview. Of course, I'd be happy to do it because I'm very interested in the position and the company. But at the same time, I wanted to ask if the reason there are so many interviews is because the company has many people applying, or if you guys are looking for something specific from me I'm not showing? And if I may ask, will there be more interviews down the line?"

This is just a rough outline of what to say. Definitely edit it however you think is appropriate.

You could also just ask. "So I was curious if this going to be the last round interview, or would there be more around the corner?"