Creating a truth table for two 4 bit numbers A[3:0] and B[3:0] for inputs and an output that is 1 when A and B are equal and 0 when false
You will have 256 lines in your truth table. 


When B is 0000, A could be any of 0000 to 1111. 
When B is 0001, A could be any of 0000 to 1111. 
When B is 0010, A could be...


Well you get the point.

The output will only be 1 when A equals B which will be in 16 different cases.