LR4 - Not backward compatible
Why would you use a beta if you don't use the upgraded features.
Sure, we try them out in the new software. But we don't expect to be able to try them out in the old software too.
And: with some "sandbox" (copied) data, and a disposable separate Catalog. "Not for production", as Adobe put it - which I interpret as, keep well away from your current production DATA.
Safe and reliable backward compatibility with LR3 - and a clean up-conversion of LR3 catalogs to LR4 - will certainly be important considerations in due course. But even then, IMO backwards compatibility only truly makes sense within a single PV ; one which the older software can understand natively. People who require that, will simply have to forego the new PV for now, until such time as they are ready to commit entirely to it.
Excellent to be able to do this image-by-image though. I did bring ALL my images up to the new PV last time, but doubt I will do so this time - unless the automatic re-interpretation of the settings, is a REALLY smart one.
So a LR3 image will continue being a LR3 image, and new imports will be LR4.