Use one of the backup LR 4 catalogs - you get prompted every week to create one when using Lightroom, so even if you sometimes hit 'skip' you will have let it run through a few times and have something you can go back to?
Also, does the conversion process itself not create a backup? Looking at my own directory, it looks like the conversion process for upgrading from LR4 to LR5 didn't actually change the LR4 file, rather copied it and updated that. In fact I have Lightroom 5, 4, 3, 2, and what is presumably a Lightroom 1 catalog file still there.
Actually, I've just opened up LR4 and LR3 on this machine, and their catalogs are intact and load by default, even though I updated to LR5 on release, so I suspect the catalog may not be your issue.
Have you reorganised your library within Lightroom 5? Moved your folders around?
When you say you have 'nothing in LR4' do you mean that the catalog is completely empty, or your links to the pictures have been broken (ie. you can see the thumbnail, but when you try to view / edit, it's missing?)