What is the OST at episode 156 of Naruto Shippuden at 4 minute mark?
This is an unreleased track. I found a version of it on YouTube titled "Peaceful", but it's incomplete and has sound effects in it. There might be other versions online, but I didn't find any others in a cursory search.

(To make things more confusing, there are at least a couple of other unreleased tracks that have been given names like "Peaceful" and "Peace" and "Peaceful theme" too...)