Had to build a new PC to replace an 11 year old HP, and was surprised that LR4 let me import all of my LR3 images without doing an Export as Catalog,from LR3,
which is how I upgraded my catalog from LR2 To LR3.
One of Jeff's tips, I took to heart, after upgrading to LR4 was to create a snapshot of an image,in develop,before clicking the upgrade to PV2012. That way you can preserve the older settings better, if you want.
Basically don't blindly update all of your older images at once, especially your finished images.
The other thing I did was to go through my file structure,from LR, and cleaned it up the way I wanted.I waited to do this on my new PC, only because an 8 core processor with 32 gigs of ram and Win 7 64 really trumps a 2gig Pentium 4 XP machine.But I currently only have a C: drive for now and wanted to reorganize my catalog and file structure to migrate to a drive dedicated to my photos.
Seems to me we are getting really close to the launch of LR5, you might just want to wait rather than upgrade now to LR4.
As for all my LR settings all I had to do was copy the LR settings folder from LR3 and replaced the LR4 folder with it.
The only things I had to redo were 2 publish to Hard Drive folders that didn't carry over, not real sure why that happened but it wasn't much to set them up again, and export my keyword list from LR3 so I could import the list into LR4.
Hope this helps
Best of Luck