If it will shift into any other gear, your issue is with the shift position sensor or something mechanical. To narrow it down, use the manual actuation of the shift motor in the diag screen. The combine engine will have to be off to do this. Have somebody watch the shift position on the indicator on the top of the trans while you press the touch screen to manually actuate it. Watch the amp bar graph, when it goes way up, let up on the button immediately - you will blow a fuse if you hold it too long. It should be possible to move it from 1st to 4th this way. If it will not move all the way, it is most likely mechanical and requires internal investigation. If you can get it to move all the way, both ways, check the shift position sensor (the kidney bean shaped thing on top of the transmission). If it doesn't sense that it is in a gear, it won't let you go in that gear to protect the transmission, and for safety. These do go bad. They are comprised of five separate switches and one can go bad while the others continue to operate.