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."
iPad Mini 3 • iPhone • B&W P3 • Mac Mini • Apple TV • Quad Vena • Audio-Note AX-Two
Is it Compatible with 4s also?
Yes. I'd suggest using wi-fi as it can be data heavy. Still my favourite app.