网址：https://hdlbits.01xz.net/wiki/Exams/m2014_q4k
自己写：

module top_module (
    input clk,
    input resetn,   // synchronous reset
    input in,
    output out);
    
    reg 	r_in1;
    reg		r_in2;
    reg		r_in3;

    always@(posedge clk)begin
        if(resetn == 1'b0)
            begin                
            	r_in1 <= 1'b0;
            	r_in2 <= 1'b0;                
            	r_in3 <= 1'b0;                
            end    
        else 
            begin                
            	r_in1 <= in;
            	r_in2 <= r_in1;                
            	r_in3 <= r_in2;   
                out <= r_in3;
            end 
    end
 
endmodule

第二种方法：

module top_module (
    input clk,
    input resetn,   // synchronous reset
    input in,
    output out);
    

    reg [3:1] q;
    
    always @ (posedge clk)
        begin
            if (~resetn)
            {q,out} <= 4'b0;
           else
               begin
                   q[3] <= in;
                   q[2] <= q[3];
                   q[1] <= q[2];
                   out <= q[1];   
                   //{q,out} <= {in, q};
               end         
        end

    
endmodule