Is it normal to wake up after 4 hours?  

I think most people do wake up after 4 or 5 hours but only some of them realize it is happening. I know when this started for my brother, he found out it was because his body had been waking him up to pee so he stopped drinking stuff 4 hours before bed and it solved the problem. He didn't feel the need to go when he was waking up but his body signaled for it anyway.

I wouldn't say it is normal but it is common. I think if people took better care of themselves (physically, mentally, and spiritually) their sleep would be as good as a child's again.

Some say it is, some say it isn't. I think if you used to sleep through the night and now you don't, you should probably figure out what is giving you issues. Maybe your bed is causing back pain?