If you want to use an external script/application that will change the default printer and use it in your 4D application, you will face the followign problem:
When the form loads, 4D will check the system to obtain the default printer settings and use this printer for the form. If the status of the default printer is changed outside of 4D, it will have no effect on the current form since 4D stores the original default printer settings.
In order to have 4D print to the printer of your choice, the changes must be registered in 4D. Instead of using an external script or application to change the default printer, simply call the Print Settings command before issuing a Print form command.