Upon installing and loading beta 4 for testing I found that it immediately crashed out with an error (rather, it kept loading, but refused to do anything and the splash screen wouldn't go away until I killed the process). While I can't recall exactly what the error is (I stupidly forgot to write it down before fixing the problem) I eventually found out (after uninstalling, deleting, reinstalling, redeleting, reinstalling in a different directory) that the problem was fixed when I deleted C:\Documents and Settings\Application Data\FeedReader
The gist of the error message was that there was a database problem and it was getting 5 pages when it was expecting 7 and that this was occuring on page 200-something (IIRC.. might have been 300 something).
Terribly unhelpful I know, but at the very least I'd try re-testing with some old installs to see where the data issue might be.
Additional questions...
What was the previous version before beta4? Was it Beta 3 or some earlier version?