You are possibly linking to a stylesheet that doesn't exist. If it does exist then make sure you are linking to it properly.
You must make sure the path is also correct if you are linking from within directories and not just from top level. Check if things work ok from the web root, If the error appears from a page within a directory then you have to fix the link to the stylesheet.
Most browsers ignore a missing or incorrectly linked stylesheet but Netscape4x treats it as a document it can't find (which is technically correct). If an image is wrong then you get a broken image marker so why complain if a browser points out a missing file (such as a stylesheet)
Before you blame Netscape 4x - check your code :)