4 Bit RCA 

module full_adder(

    input  A,

    input  B,

    input  Cin,

    output S,

    output Cout

);

    assign S    = A ^ B ^ Cin;

    assign Cout = (A & B) | (B & Cin) | (A & Cin);

endmodule

module rca_4bit(

    input  wire [3:0] A, B,   // 4-bit operands

    input  wire Cin,           // carry-in

    output wire [3:0] S,       // 4-bit sum

    output wire Cout           // carry-out

);

    wire C1, C2, C3;

    full_adder FA0 (A[0], B[0], Cin, S[0], C1);

    full_adder FA1 (A[1], B[1], C1,  S[1], C2);

    full_adder FA2 (A[2], B[2], C2,  S[2], C3);

    full_adder FA3 (A[3], B[3], C3,  S[3], Cout);

endmodule