I'd go with "THE OTHER GUY" lol

Thank you for help. I will keep you informed about results.

Drive the car at about 45 to 50 MPH, back off the gas pedal and move the shifter lever to neutral, then step on the gas and at the same time move the lever to drive, if the transmission shifts into 4th gear, it’s a struck valve in the valve body.

I have tried everithing you suggest, but still the problem is not resolved.