Samsung phones use this wonderful tool called Odin. If you get a MetroPCS phone and use Odin on it using the files from a Sprint device then you'll effectively flash the device over to Sprint. It'll act just like a Sprint device. Other than the MEID etc. If you flash it over to Boost using the usual suspects then you'll have a phone that think's its on Sprint but using Boost and was originally a Metro phone.
I've done this on Sprint Siis. Flash it to Boost and then put Boost firmware on it using Odin. The phone thinks its a Boost phone and everything works "out of the box". Everything except 4g which I have no way to test even if it did work.
The Sii, I've done myself. So it can be done. About the S4, I'm not so sure since I haven't got a device to try it on and so it is pure theory. Good luck.
[Mod Note: more than luck is needed; as noted a few posts below the MetroPCS S4 is NOT CDMA therefore flashing a CDMA-based firmware (such as that of the Sprint S4) will likely render your device in-op]