Tender 4 on the register can be defined to run an external app when pressed.
This can be any credit card app like AprivaPay, First Data, GoPayment or just about any other Android app that a credit processor has.
Once the card is processed in this external app, return back to the Registroid screen (by either backing out of the credit card app, or holding the Home button and touch Registroid) and press the Tender 4 button again. This will finalize the sale.
The "Tender 4" button can be labeled anything you want.