94 supreme tranny drops from 4th

My 94 Supreme convertible will go into 4th (overdrive?) then after a couple miles will drop back to 3rd and not go back to 4th. I can switch from D to 3rd and back with no difference. Interstate driving at almost 4000 rpms is killing my gas mileage. When 4th is engaged it runs about 2500 rpms. Any ideas?

Reading a post further down I found this...

It is possible the tcc solenoid is overloading the pcm quad driver which will kill the circuit to the solenoid and out of o/d. This should be scanned with a trans capable scanner to look at solenoid commands and operation to really diagnose this properly.

So that translates to take it to a tranny shop....answered my own question thanks anyways.