There's no helping NN4x, it's to old. You can get IE6 win, IE5+ mac, NN6 and Opera to all read the same by making your pages strict.
replace <html> with this:
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
At first you'll probably be alarmed by the results but once you clear up the CSS mess most of your x browser headaches will be gone.