Check out the below code for 4 bit Carry select adder
module carry_select_adder_16bit(a, b, cin, sum, cout);
input [15:0] a,b;
input cin;
output [15:0] sum;
output cout;
wire [2:0] c;
ripple_carry_4_bit rca1(
.a(a[3:0]),
.b(b[3:0]),
.cin(cin),
.sum(sum[3:0]),
.cout(c[0]));
// first 4-bit by ripple_carry_adder
carry_select_adder_4bit_slice csa_slice1(
.a(a[7:4]),
.b(b[7:4]),
.cin(c[0]),
.sum(sum[7:4]),
.cout(c[1]));
carry_select_adder_4bit_slice csa_slice2(
.a(a[11:8]),
.b(b[11:8]),
.cin(c[1]),
.sum(sum[11:8]),
.cout(c[2]));
carry_select_adder_4bit_slice csa_slice3(
.a(a[15:12]),
.b(b[15:12]),
.cin(c[2]),
.sum(sum[15:12]),
.cout(cout));
endmodule
//////////////////////////////////////
//4-bit Carry Select Adder Slice
//////////////////////////////////////
module carry_select_adder_4bit_slice(a, b, cin, sum, cout);
input [3:0] a,b;
input cin;
output [3:0] sum;
output cout;
wire [3:0] s0,s1;
wire c0,c1;
ripple_carry_4_bit rca1(
.a(a[3:0]),
.b(b[3:0]),
.cin(1'b0),
.sum(s0[3:0]),
.cout(c0));
ripple_carry_4_bit rca2(
.a(a[3:0]),
.b(b[3:0]),
.cin(1'b1),
.sum(s1[3:0]),
.cout(c1));
mux2X1 #(4) ms0(
.in0(s0[3:0]),
.in1(s1[3:0]),
.sel(cin),
.out(sum[3:0]));
mux2X1 #(1) mc0(
.in0(c0),
.in1(c1),
.sel(cin),
.out(cout));
endmodule
/////////////////////
//2X1 Mux
/////////////////////
module mux2X1( in0,in1,sel,out);
parameter width=16;
input [width–1:0] in0,in1;
input sel;
output [width–1:0] out;
assign out=(sel)?in1:in0;
endmodule
//4-bit Ripple Carry Adder
module ripple_carry_4_bit(a, b, cin, sum, cout);
input [3:0] a,b;
input cin;
output [3:0] sum;
output cout;
wire c1,c2,c3;
full_adder fa0(
.a(a[0]),
.b(b[0]),
.cin(cin),
.sum(sum[0]),
.cout(c1));
full_adder fa1(
.a(a[1]),
.b(b[1]),
.cin(c1),
.sum(sum[1]),
.cout(c2));
full_adder fa2(
.a(a[2]),
.b(b[2]),
.cin(c2),
.sum(sum[2]),
.cout(c3));
full_adder fa3(
.a(a[3]),
.b(b[3]),
.cin(c3),
.sum(sum[3]),
.cout(cout));
endmodule
/////////////////////
//1bit Full Adder
/////////////////////
module full_adder(a,b,cin,sum, cout);
input a,b,cin;
output sum, cout;
wire x,y,z;
half_adder h1(.a(a), .b(b), .sum(x), .cout(y));
half_adder h2(.a(x), .b(cin), .sum(sum), .cout(z));
or or_1(cout,z,y);
endmodule
// 1 bit Half Adder
module half_adder( a,b, sum, cout );
input a,b;
output sum, cout;
xor xor_1 (sum,a,b);
and and_1 (cout,a,b);
endmodule