module fadd(sum, cout, a, b, cin);
  output sum, cout;
  input a, b, cin;

  assign sum = a ^ b ^ cin;
  assign cout = a & b | b & cin | a & cin;
endmodule

module ripple4(sum, c4, c3, c2, c1, a, b, c0);
    output [3:0] sum;
    output c4, c3, c2, c1;
    input [3:0] a;
    input [3:0] b;
    input c0;
    
    fadd bit0(sum[0], c1, a[0], b[0], c0);
    fadd bit1(sum[1], c2, a[1], b[1], c1);
    fadd bit2(sum[2], c3, a[2], b[2], c2);
    fadd bit3(sum[3], c4, a[3], b[3], c3);
endmodule