使用Verilog写一个4位流水灯，并再写一个对应的仿真测试文件

Sure! Here's an example of a 4-bit shift register implementation using Verilog:

module shift_register (
  input wire clk,
  input wire reset,
  output wire [3:0] led
);

  reg [3:0] shift_reg;

  always @(posedge clk or posedge reset) begin
    if (reset)
      shift_reg <= 4'b0000;
    else
      shift_reg <= {shift_reg[2:0], shift_reg[3]};
  end

  assign led = shift_reg;

endmodule

And here's an example of a corresponding testbench for simulation:

module shift_register_tb;

  reg clk;
  reg reset;
  wire [3:0] led;

  shift_register dut (
    .clk(clk),
    .reset(reset),
    .led(led)
  );

  always #5 clk = ~clk;

  initial begin
    clk = 0;
    reset = 1;
    #10 reset = 0;
    #100 $finish;
  end

endmodule

In this example, the `shift_register` module represents the 4-bit shift register. It has inputs for clock (`clk`) and reset (`reset`), and an output for the LED values (`led`). The shift register shifts its contents to the left on every rising edge of the clock, except when the reset signal is active.

The testbench module, `shift_register_tb`, instantiates the `shift_register` module and provides stimulus for simulation. It toggles the clock signal every 5 time units and asserts the reset signal initially for 10 time units. Finally, it finishes simulation after 100 time units.

You can use a Verilog simulator like ModelSim or EDA Playground to run this code and observe the behavior of the shift register.