4 bit subtractor using 2's complement of an adder
RYour expressions for individual bits shouldn't use the term "B+1", that can have three possible values (0,1,2) and require two bits to express.

Your expressions for all 4 bits of the adder should be the same: 

Sn    = An^~Bn^Cin;
Coutn = (An&amp;~Bn)|(Cin&amp;(An^~Bn));

Where 'n' represents the bit number.

The Cin for the entire adder should be set to a 1 to do the complement + 1 function.

Try this approach to thinking about it:

Forget about A for the moment.  How would you create the 2's complement of B with your logic?

You would put the complement of B into the 4 input lines of the adder, then to add the 1 you would set the carry in.  This should result in the 2's complement of B at the output.

Now add A into the other 4 input pins.  You should get the A-B at the output.  The carry output is then used as an overflow status bit that tells you about the relative magnitude of the two operands.