How do you plan to solve it?
module pwm4_basic( input clk, rst, input [3:0] duty, output reg pwm_out ); reg [3:0] cnt; always @(posedge clk) begin if (rst) begin cnt <= 4'b0; pwm_out <= 1'b0; end else begin cnt <= cnt + 4'b1; pwm_out <= (cnt < duty); end end endmodule