In taking a further look at Fireworks CS4 beta, I have come across some compatibility issues between the previous version and this one. Hopefully this issue will get sorted out as development continues with Fireworks CS4.
Recently I was designing a final version of a web site, all pages and content included. Upon reopening the design in CS3, I found that all of the text had strangely changed values. The kerning, spacing and point size of the text that changed by fractions of a value, and even the type itself was altered. Any word above about 8 characters got shortened. Furthermore, I had some lines of text that had two different colors in the same text box, and they were changed to the first color. Finally, I had used Arial font in the CS4 version of the final, and upon reopening the file, CS3 thought Arial (somehow replaced by Arial MT) need to be replaced, so I replaced it with Arial (considering that I did not have the choice and never had had the choice of selecting Arial MT). So in a nutshell I was forced to replace Arial with Arial, which may have caused the problems listed above.
So obviously CS4 is not yet compatible with previous versions of Fireworks, and CS4 really should only be used for testing until the actual release.