How do you plan to solve it?
module pwm4_basic( input clk, input rst, input [3:0] duty, output reg pwm_out ); reg [3:0] cnt; always @(posedge clk) begin if(rst) begin pwm_out <= 1'b0; cnt <= 1'b0; end else begin cnt <= cnt + 4'd1; end pwm_out <= (cnt < duty); end endmodule