The car doesn't have a 4th gear, simple as that. The 4th gear clutch shaft is likely stripped out, common problem as they're made of a soft metal. ZZP sells hardened ones to correct it, but of course the trans has to be disassembled to correct the problem.
Try this. While cruising at 70-75 or so, let off the gas and see if the RPM's go back down to idle range. If they do, you don't have a 4th gear for sure. RPM's should go down to ~1k when you let off the throttle at hwy speeds.
EDIT: The only vac controlled trans in a GP is the 4T60E in the '97 GT's.
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XP'd and SSIC'd, 'nuff said