module full_adder_4bit (a, b, cin, sum, cout);
   input[3:0] a, b;
   input      cin;
   
   output [3:0] sum;
   output 	cout;
   wire c1, c2, c3;
   

   // instantiate 1-bit adders
   
   full_adder FA0(a[0],b[0], cin, sum[0], c1);
   full_adder FA1(a[1],b[1], c1, sum[1], c2);
   full_adder FA2(a[2],b[2], c2, sum[2], c3);
   full_adder FA3(a[3],b[3], c3, sum[3], cout);
 
endmodule