The sending unit is only half of the equation. The other half is the actual gauge, and that can also have an error. There should be a test for the gauge itself, using a known resistance across the wiring terminals for the sending unit. I don't have a shop manual for a late model bike, so I can't tell you if Harley has such a test. As far as I'm concerned a 4 psi error when compared to a calibrated mechanical gauge is excessive. Knowing how Harley does things however, I imagine they will say it meets their specifications and tolerances.
Jerry