Yep, think of a scent trail as a phosphor trace. As the target moves across the terrain, the trail they leave behind fades out over time. However, if a dog can catch the tail of the trace, which as you say decays in roughly 4 hours, then they can speed up the recovery considerably. All they have to do is cross that line once or twice to get the orientation, and then they can home in on it. Dogs are limited, but they do certainly help when used properly and within their known limitations.