I may be a bit old-hat, but whenever I see the number 404, the first thing that comes to mind is a 404 NOT FOUND error. To that end, I think it would be funny to change the Operator Level shape for level 404 to the logo of a well-known web browser.
I suggest Safari. While I am not a Mac user myself, the Safari logo is pretty recognizable, and easily constructible in the Shapez 2 quad system. Moreover, given the Safari logo's representation of a compass, it would be even more appropriate to the joke, as encountering a 404 can be considered the online equivalent of getting lost.
I would like to officially suggest the shape code CuCuCuCu:CbCbCbCb:Sr--Sw-- to be assigned as the Operator Level display shape for level 404.