what causes the gear shifter to get stuck in 4th gear?

[FinallyGotOne]

I'm wondering if the clutch cable is going. I was one the hiway today and was cruising along in 5th, i downshifted to 4th and stayed there for a couple miles. then i went to shift to 3rd and the gearshifter would not come out of 4th. I depressed the clutch a few times and eventually after a couple times it did shift out of 4th. ?? what is this issue???

1979 911 sc