If you want to quadruple your amp hours without modifying the hardware, just treat 25% of the voltage gradient of each battery as your unit of measurement.

Alternatively, since this is USB-C, and we assume the marketing copy is honest, use the max voltage USB-C can deliver: 20v.

So, draw 20V from this device and measure the amp hours it outputs.

Wh is really the only sane way to go.