module add4 (
    input wire [3:0] a,
    input wire [3:0] b,
    input wire Cin,
    output wire [3:0] y,
    output wire Co
);
 
    wire Co_temp;
 
    add2 add2_inst1 (
        .a(a[3:2]),
        .b(b[3:2]),
        .Cin(Co_temp),
        .y(y[3:2]),
        .Co(Co)
    );
 
    add2 add2_inst2 (
        .a(a[1:0]),
        .b(b[1:0]),
        .Cin(Cin),
        .y(y[1:0]),
        .Co(Co_temp)
    ); 
 
endmodule