When taking a nap to enter a lucid dream...

Does it have to be exactly 4 hours after you wake up? I woke up at 7 today, ate lunch at 11 and went to bed about half an hour after that. I was in that state where I'm really relaxed and I have those random "dream flashes" but then I got a text and I can't go back. Is there a set period of time after you wake up that your REM is strongest and then it just goes away?

I believe that for most people your body goes into a natural dip between 1 and 3 pm so that should be anywhere between 5 and 7 hours of walking up.

In the morning hours, you experience much more REM sleep and deep dreaming than during the early stages of sleep. As dutchraptor suggested, 5-7 hours into sleep should be the ideal time to wake up.