I just bought an unused/unregistered copy of Dreamweaver 4 and Fireworks 4. I realize that Dreamweaver 8 is more refined and polished probably, but are there things that I can't do with D4 that I could do with D8?

Many and varied. I scoffed at DW back in the day but recently used it on 2 projects involving mysql and it has come a very long way in the last 4 years. If you can, upgrade but at the least grab the 30 day trial and compare.

Also, take a look at http://www.interakt.ro as they make some products that will substantially speed up your process if your doing db backed work.

HTH,
Iggy