Does the Phantom 4 Pro have less quality in 4K 60fps than 4K 30fps?

No, in my experience it doesn't have less quality in 60 fps.

Also depends on why you are asking and what are you actually trying to achieve ?

You could argue that 60 fps is better, especially if the video is to be slowed down for viewing or there is a lot of movement going on both from the drone panning to the actual subject in the frame etc.

Personally, if I had to film in 4k for still or landscape environments/subjects with no fast movements then lower fps would probably be fine,  because not much is changing in the images.

On the other hand, if you are zipping around and filming some action event and may want to slow the film down in post, I would think you want to use 60fps(might look a bit jerky if 30fps is used)..