The G4 can do all carriers. So, on Ting now, I want to activate my rw t-mobile sim, drive around the 6 counties and test for a few hours or days. . Can I just activate, and will the sprint ting sim work once I pop back in, or after put in and dial ##msl# or some other voodoo code?
Use of a Republic (or any other GSM SIM) shouldn’t interfere with your phone’s Sprint CDMA activation on Ting. GSM differs from CDMA, in that service is tied to the SIMs ICCID not the phone’s IMEI/MEID.
I’ve actually gone in the opposite direction of what you propose by using multiple GSM SIMs (domestic and international in my Republic CDMA activated Moto X Pure. No impact on my MXP’s CDMA activation on Republic nor is there reason to believe activation and use of a Republic GSM SIM in your Ting CDMA activated phone would have any impact on your Ting service.