It depends on the processing options of your P4310 version. If the Route Code option is set to 4 - Default Location, the approval route will be retrieved from the user record in P400951. If you have the Route Code option set to 3 - Branch/Plant, you may enter any branch/plant (that you have security to) in the PO header, and the route will be retrieved from the BP Constants P41001. This option offers more flexibility.
There is not a way, that I am aware of, to hard-code an approval route in the P4310 processing options, without customization.