I just had a master cylinder installed in my 1994 Toyoda 4runner and now my 4th gear is slipping and over reving
Would that have been the clutch master cylinder or the brake master cylinder? Either way, the master cylinder send hydraulic pressure down the the slave cylinder to move the pressure plate fingers. When your foot is off the clutch pedal, there is no pressure or there should not be any pressure. Is the clutch engaging near the top of the pedal travel? That's a sign of a worn clutch disk.