You don't have Machinery's Handbook? Shame, shame
The taper of all National Pipe threads is 3/4" per foot on the diameter, which translates into .0625" change in X for every inch of Z travel.
The effective thread length for 4" NPT is 1.300 inches. Supposing you begin .2" in front, the amount of taper in 1.5" is .09375", which is the amount you would use in your G76, and here I'm guessing, parameter "I" would be either .0469 (if I is a radial value) or else .09375 if it is a diametral value.
The pitch diameter at the beginning of the external thread is 4.33438". This can be measured using the "two wire method", if you are accustomed to using Peedee wires to check.
FYI, All pipe threads from 2.5" pipe on up, are 8tpi.