hey guys i need to make a 4 bit subtractor from scrath using logic gates.
can someone give me a boolean expression for this and a logic ciruit.

 

I could but you wouldn't learn anything. You should start with a table for a one bit subtractor without borrow. It will have four entries, one for each possible combination of 1-bit operands. You will derive two output functions: difference and borrow. From there it is easy to add the borrow condition in and you will have your answer.

 

Doesn't the building block need 2+1 inputs (not 4)? One for each bit and one for the carry.