93 ford e150 wont shift into 4th gear?

I have a 1993 ford e150 that I recently discovered won't shift into 4th gear. I figured it out after counting the gears when I noticed it was revving unusually high at 65mph (normal highway speed limit in my area) I checked the transmission fluid and it seemed to be at the right level. Could my transmission be going bad? It only has 147,000 miles on it. It sat in a lot for years before I bought it Any advice is appreciated. Thanks.

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