Post was several months old but it showed in the top results of my search, so I just want to clarify what ZIRCOBEN said in case this comes up for someone else seeking information.
ZIRCOBEN SAID: "No. It would have to be from a CDMA carrier like Sprint or Verizon. AT&T and T-Mobile have GSM networks"
For most - but not all - phones, it is true that they are compatible with either GSM (T-Mobile, ATT, Metro PCS...) or CDMA (Sprint, Verizon...), but there are SOME phones that are compatible with both GSM and CDMA, and one of such phones is the Galaxy S4. I own two GS4's with sprint (CDMA) branding, I sim unlocked both and currently use them with T-Mobile (GSM). With T-Mobile, everything works (calls, text, internet) except for 4G LTE, but you still are able to get HSPA+ internet speeds which is faster than 3G but slower than 4G LTE. I can also use any simcard from the other GSM carriers.
The correct answer to the original post is YES, you can sim unlock a GS4 and use it with Metro-PCS.