I'm not new to the Naruto franchise. I'm also up to date with what's currently going on (Madara vs Naruto & Sasuke). But I want to know the details of how everything happened. From which episode should I start watching? I want to at least start after the whole Pain storyline and around where the Fourth Ninja War starts. Anyone got an idea on where I should start?
As this is the point where the 4th ninja war starts.
It spans through volumes 55 to 59, or more specifically, covers chapters 516 to 559 of the manga and episodes 261 to 270, and also picks from 272 to 289 of the Naruto: Shippūden anime and later picks up from episodes 296 to 310, and 312 to 321. The six episodes of the Power arc, along with two others, were shown in the interim. This arc is preceded by the Fourth Shinobi World War: Countdown and followed by the Fourth Shinobi World War: Climax.
So you can either start at chapter 516 or if you prefer the anime, start at episode 296 of Shippuuden to skip the pain arc, and jump straight into the war.
I recommend starting at the beginning. You can watch the whole show on Ani.ME (legally). If you try skipping around, you'll miss a lot of the key dynamics between the characters... even if there's a lot of filler, sometimes the filler episodes contain more backstory than you'd expect.