What could cause no 3rd or 4th gear in a 4l60e?

No 3rd or 4th gear in a 4L60E transmission could be caused by several issues, such as a malfunctioning shift solenoid, low fluid levels, or a blockage in the hydraulic system. Additionally, worn or damaged internal components like clutches or bands may prevent the transmission from engaging these gears. Electrical issues, such as a faulty transmission control module (TCM), could also lead to gear selection problems. Lastly, poor linkage adjustments or cable issues might affect the transmission's ability to shift into higher gears.