How do you plan to solve it?
module bin2gray4 ( input [3:0] bin_in, output [3:0] gray_out ); assign gray_out [3] = bin_in[3]; assign gray_out [2] = bin_in[3] ^ bin_in[2]; assign gray_out [1] = bin_in[2] ^ bin_in[1]; assign gray_out [0] = bin_in[1] ^ bin_in[0]; endmodule