Hello there,
In order to unlock the Galaxy S4 for MetroPCS, you must contact them first and ask. If you have a legitimate reason, such as "I need to go overseas for a while", they will unlock it for you. 9/10 times they probably will if you have a legitimate reason.
From my knowledge, the GS4 for Metro is the first GSM phone they have. Other MetroPCS phones that are CDMA can not be unlocked since they don't use SIMs.