use something like a clapper to match up the audio between the flips sound and the h4n when editing. I used to use the h2 as a boom mic when recording audio for a friends movie. I made sure I used one of those clappers (or just bang two pieces of rock or wood together. You'll see a huge spike in the sound waves and just match the big spike from the flips sound to the spike from the h4n.
FYI I don't know if the flip records audio, but I'm assuming it does. Hope that helps.