When mine failed, I lost all but 4th. I'd pull the shifter and see what moving the shift rails by hand does. May have lost the 3-4 shift fork connection to the rail. This would mean that you can not lock 3rd or 4th to the shaft to engage those gears. Since 1 and 2 engage, I don't think you have the classic bad input shaft the km145 is known for, but it's possible...

Not responsible for advice not taken...