I've been using
this app all day and have to say it's really good. Think of it as an enhanced LastFM that uses the Spotify catalogue. It's free at the moment and only came out in the last few days.
"SpotON Radio requires a Spotify Premium membership to run."
"We are currently awaiting the loading of our complement of small lemon-soaked paper napkins for your comfort, refreshment and hygiene during the journey."
Is it Compatible with 4s also?
Yes. I'd suggest using wi-fi as it can be data heavy. Still my favourite app.