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If the transmission is slipping in 4th gear while accelerating, here is a list of possible causes.
If the check engine light is not on and the OD light is not on or flashing, this indicated that the computer has not determined that there is a fault in the system. Verify that there are no fault codes or lights on.
If one or both of these light come on, then a fault code will be stored and this will lead you to the system that is causing your concern.
If the fluid level is full and clean, I suggest that you continue to drive the vehicle until a fault code is stored in the computer or the concern happens all the time.
Otherwise a long list of diagnosis steps will need to be performed to determine where the fault lies. (fluid line pressure test, stall test, solenoid testing, wiring and module testing.)
There are no common faults for your concern. These transmissions have been very good.
Please let me know if you have any additional questions.
Engine flares up or slips when upshifting or downshifting
When accelerator pedal is depressed at start, engine speed increases normally but vehicle speed increase slowly.
When accelerator pedal is depressed while driving, engine speed increases but vehicle speed does not.