Thunderbolt normally supports multiple protocols over the same plug/cable. The Thunderbolt 2 port on my old MBP is where you plug in the monitor. Thunderbolt 2

Thunderbolt normally supports multiple protocols over the same plug/cable. The Thunderbolt 2 port on my old MBP is where you plug in the monitor. Thunderbolt 2 matters, because that comes with DisplayPort 1.2 support, which is necessary for 4k@60Hz.