tl;dr - You are MUCH better off getting a T-Mobile variant of the Note 4 if you want to use a T-Mobile MVNO. However, the AT&T model will work to a limited extent.
MetroPCS is a T-Mobile service. I use a similar service called MintSIM that offers T-Mobile without roaming agreements, and since I am in an area of decent T-Mobile coverage it works reasonably well with some pretty big caveats.
- The AT&T phone supports some, but not all, frequencies for EDGE and LTE. It supports NONE of the frequencies for HSPA. So you are basically looking at "wicked slow", "wicked fast", or "nothing". Around here I have support for both EDGE and LTE, thankfully, so it works "okay".
- The AT&T phone appears to come with some LTE bands disabled by default. I found a service dial code that allowed me to get into a menu to enable all the LTE bands the phone itself physically supports (which is not all the bands that T-Mobile uses for LTE, but this enabled the band that is commonly used around here - probably Band 4).
I bought three months of MintSIM for $35 total. I've been using it actively for about a week now and I am reasonably happy with it, especially considering that I can buy a year of it for $160 and that's CHEAP cell service for a 2GB LTE speed plan plus unlimited talk and text. However, if I decide to continue with it, I'll probably pick up a T-Mobile variant of some phone or other. It's just as likely that I will go for AT&T's $40 a month 6GB prepaid plan for a while.