Something happened to the input gear of your countershaft. I'm not sure how Chevy does theirs, but other makers use keyways to assemble the input gear to the countershaft and they can strip and the only gear left will be 4th. I hate to tell you but, I'm afraid the trans has got to come out,
Scott

Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!