Unless you plan to use it on a different planet than the Earth, a 4' wide trailer is just a non-starter. Even rural roads that are almost off-grid constrain the size of what you would pull behind a bicycle. Have you looked at motorcycle trailers? I can't see anything wider than 36" being practical. I'm sure there is stuff at Instructables.com to get you started, but you really should think about why the existing trailers on the market are ALL built the way they are.