When you save an Illustrator CS4 file with multiple artboards in native Illustrator CS4 format, the file extension is still plain old ".ai".
Earlier versions of Illustrator can open the AI file, but users will get the cryptic alert "This file was created in a newer version of Illustrator. If you import this file, some data loss may occur." When they open the file, they'll see only one artboard, the one that was labelled "1" in the original file. There is NO indication that there's a bunch of artwork missing from missing artboards.
You'd think that Adobe could've had given the hapless end user a little stronger warning, hmm? Something like, "This file contains multiple artboards. You will only see one." I mean, they were able to sneak in a little warning about the type engines changing between versions (remember Legacy text?), why couldn't they sneak in a warning about artboards? Sigh.
The lesson is this: If you're a fan of multiple artboards — and who isn't — but you sometimes need to share native AI files with users of earlier versions, you'll need to do some extra work.
Don't Miss the Artboards Checkbox in Save As
When you choose File > Save As in Illustrator CS4, and then from the dropdown Format menu choose EPS or PDF, extra controls light up in the Save As dialog box. Don't click the Save button yet, pay attention to these controls!
For example, if you choose the EPS format, the checkbox for Use Artboards and the Page fields become enabled. If you uncheck Use Artboards, then Illustrator creates one huge EPS containing all the artboards in their current locations.
Do you just want one EPS per artboard instead? Then turn on the Use Artboards checkbox, and specify which artboards (Pages) get exported. Illustrator will create one EPS file for each artboard, appending "01" and "02" to the filenames automatically.
When you Save As a PDF, the Use Artboards checkbox dims (but is checked), because it can't create one huge PDF encompassing all the artboards. You need to enter something in the Pages fields, which are enabled. Choosing All Pages, for example, would create a multi-page PDF out of your multi-artboard Illustrator file.
Saving in Earlier AI formats
What throws many Illustrator users off is this: Choosing Illustrator as the Format in Save or Save As — in other words, creating a regular old AI file — dims the Use Artboard and the Pages controls in the Save/Save As dialog box.
It's only when you get to the next dialog box, Illustrator Options, and choose a pre-CS4 Illustrator version from the dropdown menu at the top, that these controls reappear. A new checkbox labelled Save Each Artboard to a Separate File lets you do just that. There's a Page Range field as well so you can specify which artboards to save.
If you leave the Save Each Artboard checkbox unchecked, Illustrator will include all the artwork in every artboard when it saves the earlier-version AI file. What's cool is that it then converts each artboard's borders into guides, and neatly stores them in a bottom "Guides for Artboard" layer in the Layers panel. Twirl open the layer and you'll see multiple sublayers, one for each artboard's guides.