module full_adder(a, b, cin, s, cout);

input  a, b, cin;
output s, cout;

assign s = a^b^cin; 
assign cout = (a&b) | (b&cin) | (a&cin);

endmodule

module RCA(p, q, ci, r);

input  [3:0] p, q;
input  ci;
output [4:0] r;

wire   [2:0] carry;

full_adder fa0(p[0], q[0], ci, r[0], carry[0]);
full_adder fa1(p[1], q[1], carry[0], r[1], carry[1]);
full_adder fa2(p[2], q[2], carry[1], r[2], carry[2]);
full_adder fa3(p[3], q[3], carry[2], r[3], r[4]);

endmodule