IPhone 4 screen goes blurry?

Whenever I'm watching YouTube on my iPhone 4, the screen goes blurry so that I can't see it. Does anyone know what's wrong with it.

That's the YouTube video quality, not the screen.

It could be your internet speed or youtube settings.

That would be due to the quality of the youtube video, not your screen. Usually video quality is set automatically, depending on connection speed. Try using a better connection, that should solve the problem.