mjb, consider yourself lucky that your gauge reads too low rather than too high like many have. Mine nearly stranded me on the initial tank of gas; gauge read 3/16 and the bike started to hiccup and almost stalled. Managed to pull into a gas station before it quit, took 5.4 gallons to fill my 5 gallon tank.
There is no acceptable reason for the gauge to be that inaccurate. Have it repaired/replaced under warranty, after you verify that you really got it full when you put in 4 gallons. Some folks have had issues with the 6 gallon tanks trying to get them filled completely. Best approach, drain the tank (syphon), add one gallon and then see what the gauge reads. Continue to add one gallon at a time and note the gauge reading each time. This should give you a truer picture of the "accuracy" of your gauge.
Jerry