4 bit full adder verilog code

module full_adder_4b(sum,cout,a,b,cin);
   output [3:0] sum;
   output     cout;
   input [3:0]     a,b;
   input     cin;

   full_adder f1 (sum[0], cout0, a[0], b[0], cin);
   full_adder f2 (sum[1], cout1, a[1], b[1], cout0);
   full_adder f3 (sum[2], cout2, a[2], b[2], cout1);
   full_adder f4 (sum[3], cout, a[3], b[3], cout2);

   endmodule