Yes, it's backward-compatible, BUT you have to manually import the data. I say "manually" because the system will do it for you, you just have to hit the button that tells it to do so. I did this intentionally only because there was such a delay in getting this out. I thought some people might have upgraded already to v4 a while back and may not necessarily want the old data coming back in.
So in short, yes, it's compatible, you just have to push the button.