Like StickyEmu said, you may be reading it as 5 ppm and be closer to 8 ppm. I would try a 50% water change and a new set of readings. The color change between 4 and 8 ppm is almost something that I can't discern. If you are starting at a 5 , you should be able to see it come to around 2 with that 50% water change and that would be plenty to get things moving. If it is too high, it will get you closer to the ideal starting point which is also something worthwhile doing. At least you will know whether or not you have been reading things right.
