I seem to at least wake up momentarily at around 7 am each morning but I could never wake up at 4am on my own.
Looking at some of the theories that have been aired so far I think I've pasted together a decent one.
Being the warrior sort, Merry drank lots of water before going to bed, then had a nightmare about a rather unpleasant flood. He was rudely awakened at 4 o'clock the next morning & discoved that only part of his nightmare was a dream. He then decided to never again drink so much water before bed & later went on to invent the Shire's first alarm clock.
That's the best explanation I've got.