It looks like your sychromesh between 3rd & 4th gear is on the way out. From
previous postings, I could see this is a fairly common problem. The right way
to fix this problem is to have the tranny rebuilt (expensive). My solution (I
have the same problem) is to double clutch when shifting and shift slowly and
gently, and fill the tranny with synthetic
gear oil. So far I have been driven like this for over 10,000 miles without
sticking in any gears again. 
This solution is taught to me by a very sympathetic mechanic (in New York
City!) who also taught me how to double clutch.

Worth a try.
Good Luck.