How to determine the DC operating point of a GaAs RF HEMT
Pinchoff is defined in that spec as Ids=0.1*Idss, i.e. 24mA. But that's at Vds=1.5V. 

As it's a transconductance device, you could expect Ids to be roughly proportional to Vds, or about 4/1.5*24mA or about 64mA at the higher Vds. 

So you're looking at the difference between 64 and 80mA between those two specifications, and I'd be surprised to find much closer agreement than that from a simple linear analysis of a not very linear device.

Why that disagrees with the Shockley equation I don't know, perhaps that relied on a different definition of the pinchoff voltage?