You can not adjust for both of them.
First is to the far right & 4th is to the far left
The usual reason for failure to get first is an engine idleing too fast.
50 rpm will make all the difference
If the lay shaft has too much end float the you can get failure to select both 1st & 4th

I just rebuild my gearbox.
It work correctly till i connected the adjusting arm to the gear selector...especialy in 4th gear it was very bad..after good looking why i discovered lots of wear in the clevis pins and forks...i re-drilled the arm , gear selector and both forks to a 0.5 mm oversize and made 2 new pins...result ...no wear between the arm and the gear selector anymore wich result in perfect shifting now.