4
Couple things needed to know:
1. What version of each browser is being used for testing?
2. What theme are you trying to use?
3. Can you take a screen shot to see what looks incorrect
It is well documented that Netscape 4.x does not work with CSS based sites well (sometimes not at all). If your user base requires the use of Netscape 4 (corporate environment, significant percentage of users refuse to upgrade, etc.) then your best option is to design a custom theme that does not utilize CSS for layout, only for font styles, colors, etc.
You would probably also have to customize the templates used by many modules to achieve a consistant look throughout the site. The smarty template engine is flexible enough to accomodate this, but the default theme, and most freely available themes, have been designed with modern browsers in mind.
IE 5.5, IE 6, Opera 7, Mozilla, Netscape 7, Firebird, Safari (just to name a few) all fall into this category.