Perhaps a moderator will move this to the TRANSMISSION section instead of the TRANSFER CASE section.
The only solenoid in the '90 4L60/700 transmission is for the lockup torque converter. Are you sure the problem is between 3rd gear and 4th gear, and not the torque converter locking and unlocking?
If the torque converter is locking and unlocking, it MIGHT be normal. You could verify the adjustment of the Throttle Valve (TV) cable. Connect a scan tool, see what the computer is commanding for torque converter clutch operation. My torque converter unlocks above 75 mph, lower speed if there's a headwind, a heavy load, uphill, or I'm accelerating. If this is really 3rd gear--4th gear problems, I'd be wondering about the 2--4 band or the 4th gear servo / accumulator part of the 2--4 servo assembly. The band and servo seems to be a weak point of this trans, but there's other possibilities.