My cat is 4 and he won't eat at all and he is becoming very skinny he doesn't meow anymore and about 2 days ago he was breathing really fast. What's wrong with my cat?

You need to take your cat to the vet right away. Usually heavy breathing is a sign of stress. Your cat is very sick from the sounds of it.

Right away means yesterday. You're chancing liver disease if he doesn't already have it. Something caused the cat to stop eating, so you not only need to find out what, but you have to get the cat eating cause a cat not eating can lead to death. i.e. we're not kidding and this is serious. Never ignore a non-eating cat.