Hello

I send this message to advise you that your schema of "binary multiplier" is incorrect !

Why ? Because on your schema c0 = b0.b1 and not a0.b0 !

I think that your fault can be simply correcty in writing "a1a0" instead of "a1b1" and "b1b0" instead of "a0b0".

You can find correct schema on Wikipedia on

https://en.wikipedia.org/wiki/Binary_multiplier