新手求助，急求！

哪位大虾能告诉我这程序问题出哪里啦啊?

语法没错误，但是综合就报错啊！而且我也不知道咋改？

做的是把四位加法器串成16位加法器

module adder
 (A,
 B,
 cin,
 OUT,
 Co
);
input [15:0] A;
 input [15:0] B;
 input cin;
 output reg [15:0]OUT;
 output reg Co;
wire wire_1,wire_2,wire_3;
adder_a u_adder_1
(.A(A[3:0]),
 .B(B[3:0]),
 .cin(cin),
 .OUT(OUT[3:0]),
 .Co(wire_1));
 
 adder_a u_adder_2
(.A(A[7:4]),
 .B(B[7:4]),
 .cin(wire_1),
 .OUT(OUT[7:4]),
 .Co(wire_2));
 
adder_a u_adder_3
(.A(A[11:8]),
 .B(B[11:8]),
 .cin(wire_2),
 .OUT(OUT[11:8]),
 .Co(wire_3));
 
 adder_a u_adder_4
(.A(A[15:11]),
 .B(B[15:11]),
 .cin(wire_3),
 .OUT(OUT[15:11]),
 .Co(Co));
 endmodule
 
 module adder_a
 (A,B,cin,Co,OUT
 );
input [3:0] A;
 input [3:0] B;
 input cin;
 output reg [3:0] OUT;
 output reg Co;
always
 {Co,OUT}=A+B+cin;
 endmodule