Question about 4-bit binary adder on 7 segment display and subtraction
So basically whenever you do 1-9 in binary that looks like 

00001 (1) - 01001(9) 

however since most of the subtraction is done by twos complement it is actually 

00001 + 10110 =>  10111 

Your display will only display the bottom four bits which are 0111, which is seven.

Cheers!