I've found Netscape's own documentation to be of great value. Netscape's DevEdge site has archived papers, manuals, etc... from 1997-1999 (NS4's active period).
You'll find them at:
The first link on the page, called "Dynamic HTML in Communicator' is what you are looking for. You can view it online, or download a PDF or a ZIP.
There are other links on that page to help get you going.
btw, most properties in NS4 cannot be modified after a page has achieved inital layout. Some exceptions exist though: positioning (of course), clip values, and some layer properties... The 'width' property can be used via the resizeTo() method on layers and windows, but not (for example) on images.
Anyway, the article I reference above has the necessary details.
Hope this helps -