John, I chose to keep the B3 library and installation, and let B4 make a new library. In this way I have a backwards path (nothing so far indicates I need to go back...).
Just close down LR B3, rename it in the Applications folder ("Old Lightroom is OK). Then go to your Lightroom library and rename that (to be certain, I then used a name without "Lightroom" in it; probably not necessary). Should add, I´m on a Mac.
Then I just dragged the new Lightroom to my Applications, and started it. It told me it could not find a library, and I could let it build a new one. Now, I can import a few folders of images to try B4 out in a leisurely manner. It does have to rebuild all thumbnails, but that´s WAY faster than it was in B3, and there´s only limited backwards compatibility with B3 adjustments anyhow, so you don´t lose too much work (let B3 write info to XMP sidecars before doing all this, and you can still have your keywords and other metadata).
So far I like B4 very much; instead of an "experimental setup" with B3, I now have a working tool, fast enough to really use.
A word of caution: after doing this, do not start B3 again until you have swapped names of the new library (if it is in the same place as the old one), or B3 will try to read and modify the B4 library...
