I ran iOS 6 betas on my own 4 prior to selling it and getting a good deal on a 4S, and found no performance issues compared to that of 5.1.1.
I've also installed the public release of 6.0 on my wife's 4, and it's no worse off then 5.1.1, so performance wise you have no problems or issues to be concerned with.
What I would say is that the 4 gets very few of the "major" features, such as Siri, Turn by Turn, Flyover, etc. However, Shared Photostreams are an excellent addition for sharing shots and memories with loved ones or selected friends in a private album.
This feature has proven invaluable in creating a shared album of my newborn niece, which we can all contribute to as she grows up, safe in the knowledge it's on iCloud.
Hope that helps.
What software/hadware do your 'friends' need for sharing your photostreams?