The biggest change in FeedDemon 4.1 is that the social features which relied on Google Reader have been removed. Their removal was necessary because Google announced a while back that it was dropping the social features from Google Reader.
That's the most noticeable difference in the new version, but under the hood it's also faster than ever, and it fixes a number of problems customers have reported. A complete list of changes can be found in the release notes, or you can skip all that and download FeedDemon 4.1 now.
Just install this new version directly on top of the previous version - there's no need to uninstall first.