Sounds like the 4th gear synchro is shot. But it's hard for people to diagnose sound over the internet.
Has the fluid ever been changed in the trans? If not I would start there.
Your transmission is the AX-15 speed. For fluid Redline MT 90 or Redline MTL works GREAT in your trans. Also this is a simple job you can do yourself...easier than changing motor oil. You can pick the Redline fluid up at Summitracing.com or Jegs.com or some auto parts stores.
Here is a writeup for changing the transmission fluid for your transmission....
CLICK ME--->> AX-15 Manual Transmission Service