Your dx value is far, far too low. Like, ridiculously low. Your values are not snapping from 0 to 4000. They are going in 100 ft increments. They're just going too fast for you to see. The dx value is the distance travelled, in pixels, across the screen, for the values to go from 0 to 4000. In this case, 33% the distance of 1 pixel. You're going from 0 to 4000 in LESS THAN 1 PIXEL. The drag needs to be the pixel distance. So for a high resolution screen, say 4096 pixels across, a good number would be about 2500 for the dx value, depending on the speed you're looking for. This would allow you to go from 0 to 4000 by dragging across about half the screen. The lower the number, the faster the values change due to the shorter distance required for the drag travel. Want it to go slower, INCREASE the dx value. Also, change your exp to at least 1. This will make it a linear drag, regardless of drag speed. Hope this made sense. It's getting late and I've been unwrapping and texturing for the last 16 hours.