It'll be different gates on the transistor for the different bits, and you power only one set depending on which bit of the result you wish to calculate.

Some would call it a multi-gate transistor, whilst others would call it multiple transistors in a row...

That, or a resistor ladder with 4 bit branches connected to a single gate, possibly with a capacitor in between, representing the binary state as an analogue voltage, i.e. an analogue-binary computer. If it works for flash memory it could work for this application as well.