There's a few different ways to pressurize your WaterPORT 4.0 Gallon Water Tank, but the simplest maybe the WaterPORT PumpCap. The PumpCap swaps out for the standard cap and can be pumped to pressurize the tank when you're out in the middle of nowhere.

This is the PumpCap only; the WaterPORT 4.0 Gallon Water Tank can be found here, and the WaterPORT x SeaSucker Tank Mount can be found here.