Digital-to-analog conversion of a product of digital inputs
The best option is to multiply digital inputs and feed them to an 8-bit DAC.
But if you have some spare 4-bit DACs, Opamps and resistors, it's possible to do a different way.


Feed the first digital input to the first DAC, use some constant voltage source as the reference voltage
Feed the second digital input to the second DAC and use the first DAC's output as its reference. If the DAC requires both positive and negative reference, use an opamp to invert the first DAC's output and feed the opamp's output to the negative reference input of the second DAC.