I've searched everywhere but couldn't find this music. It starts from 14:45 min in episode 4.
The song in question is track 26 of the first CD, アゲイン — meaning "again," what Kaori says at minute 15:04 to Kousei. You can buy the song individually here.
You can buy the full album in the Amazon page linked to above, but also from the show's official page if you'd prefer.