I just did my research and placed in a ticket with R+ and the Support Tech confirmed that the SIMGLW446C sim card will work with the new Moto G4 and G4 Plus. If you got the Play it might not work for it.
Thank you for contacting RingPlus support. The sim card needed for both the Moto G4 and the Moto G4 Plus is the Sprint sim card model SIMGLW446C.
If you need further assistance with this issue please reply to the ticket if not you can close it out.
If this has resolved your request, please change the status of this ticket to Closed. A screenshot with instructions is posted below.
Store To Get The Sim Card: