I'm one who has a lot of opinions, but who doesn't really know a lot. As far as your questions are concerned:
1. Lightroom 4 is an official release suitable for doing "official" work. I think it's worth it to install it if you already own it and have the serial number. If you don't, then I would suggest experimenting with the Lightroom 5 beta. It has been very stable on my computer.
2. Historically, new released versions of Lightroom have converted older catalogs. I have seen nothing to indicate that things will be different this time. The beta will not convert catalogs, but that was by design.
3. I loaded Photoshop first, then Lightroom. Didn't have any problems. I don't use any plugins.
4. This is one that I have no experience with. But my instincts would be to put it on the primary drive in the default location because that is the way the program is designed. I don't know what the implications are of installing Lightroom on a different drive. And I don't know how that impacts updates as they come along. Those who know will probably be able to tell you the answer to that one.
5. I consider myself kind of a computer idiot. I bought a new computer a few months back. Before I switched to the new computer I put all of my images and folders that were in Lightroom under one main parent folder. I copied that folder and the catalog and the previews to an external drive. Then I did my program installations on the new computer, copied my catalog and files onto the computer. The pictures were on a drive with a different letter designation than they were on the old computer. So I had to right click on the top folder and choose find missing folder. I located the folder on the new drive, everything synchronized painlessly, and I just went to work as though nothing had happened. I was expecting a lot of trouble (a lot of trouble) and I didn't have any trouble at all.
