T-Mobile has long since abandoned the traditional contract model for phone purchases, meaning in one way or another, you're paying for the entire cost of the phone. You can buy it up front or pay it off monthly, but T-mobile will get a little less of your money right now if you want to grab a new LG G4—this device is on sale for 20% off on T-Mobile's website.
The G4 is usually $599.76, which works out to $24.99 monthly on a payment plan. Right now the Tmo G4 is down to $479.76 or $19.99 per month for 24 months. That's a substantial discount compared to other carriers like AT&T ($630) and Sprint ($600). Verizon actually gets the closest with $550.
T-Mobile doesn't list any dates for this sale, but you shouldn't wait too long. It's a solid discount and probably won't last forever. T-Mobile is also the only carrier offering the brown leather back in the US, if that's something you're into. You can also take advantage of the free back cover and battery deal until the end of the month.