It means the file was created in an earlier version of Illustrator, that it contains at least one text element, and that the way Illustrator handles text is not the same as it was. It doesn't matter whether you update now or later, but there is a chance the text will change in some way such as the spacing or kerning.
If you still have the older version of Illustrator installed, open the file in that first so you can compare it after you do the update in CS4.
I've never seen any noticeable change myself but exercise caution until you're comfortable with it.