Lion runs fine on 4 GB of RAM. Again it will depend somewhat on which apps you are running and how many apps you have open at a given time, but overall it runs fine on 4 GB.
I was getting spinning beach ball a lot more after upgrading my 2010 MacBook Pro with 6GB of RAM to Lion. I wasn't getting any prior to Lion. I added another 2GB today (8GB total), no more beach ball.
Well just earlier today I was running 5 Xcode windows, iPhone simulator, 3 Photoshop projects, 8 tabs in Safari and PandoraJam, and I would get the beach ball occasionally. Runs pretty well on my 2011 13" MBP with 4GB of RAM.