Good discussion on DVinfo.net of the Zoom H4 recording offset problem.
A recording offset is the difference in length (offset) between two recordings--in this case, the one from your digital video camera and the one from the zoom.
Recording offset only matters when you are trying to synchronize recordings from two devices--usually an audio recorder and a digital video camera. If you are only recording sound with one device, offset doesn't matter.
Short version: it looks like all 'consumer grade' digital recorders have some level of recording offset. The zoom is better than most.
Cheap fix: run a simple test on your Zoom H4 (described here) to see how much recording offset there is in your zoom after 1 hour of continuous recording. Then stretch the zoom audio in an NLE (Non-Linear Editor) accordingly.