Simple, Whenever you want to add in a new digit, first multiply your accumulated value by 10 or 16 (whatever number system you are using.) Repeat this for each digit.

But my question is:
Why do you need a 4 to 16 multiplexer when only using 16 keys? Usually 16 keys are in a 4 x 4 matrix, so your 4 lines going to the multiplexer could just as well go to the keyboard.

<>< Lance.