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.
replySome 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.
replyThat's much more informative, I think my original comment is quite off the mark then.
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