You are right it takes 4 hours to have one sleep cycle. You can get waken up earlier, but if you are in Phase 3 or Phase 4 your brain will be creating gamma waves (or alpha can not remmeber which ones), that is why you see people wake up and are out of it all day
Most adults only get 1 sleep cycle, where children will undergo 2 usually
If I remember right
Phase 1 is falling to sleep, this is the euphoric feeling you get when you relax when you are real tired, lucid dreaming can happen here also
Phase 2 still producing alpha waves and beta waves
Phase 3 transition
Phase 4 REM sleep