Tello is now offering 4-SIM activation kits on Amazon, $7 with free Prime shipping.
https://www.amazon.com/Tello-Mobile-Bring-Your-Phone/dp/B07Z99VXX2
The supply of the Sprint-labeled activation kits seems to have dried up, at least on Amazon.
My assumption is that this kit will have the same 4 SIMs as the Sprint 4-SIM kit for unlocked phones, and that they will be usable anywhere on the legacy Sprint network.