Is the accuracy of the 4480 the percent of range like the Model 43 or percent of reading like the 4410A or APM-16?

The 4480A will give you 4% of reading measurements. This means the if you have it side-by-side with a 43 that is attempting to measure 200 W using a 250H (or 250 W) element, the model 43 will have limits of +/-12.5 W whereas the 4480A will have limits of +/-8 W, no element needed. 

The 4410A will provide ±5% of reading for any reading above 20% of the Power Range selected for FM or CW signals without AM.

The APM-16 will provide an accuracy of +/-5% of reading.