1993 Subaru Impreza Question: Transmission problem – Gear change

My car is Subaru Impreza 1993 AWD at about 150,000mls.
It’s been having a same problem ever since I purchased it about 4 years ago, but lately it’s getting worse.
The problem is gear won’t change smoothly. RPM goes up to 4,000 then the gear changes.
RPM doesn’t go up and down it just doesn’t change gear at normal RPM is all.
I would greatly appreciate your advices!
Thank you!