The four armed minifig
The good news is: This Minifig is still available (as of March 2017)! The bad news is, he's only in two sets and they're both quite expensive. He's Nadakhan, the captain of the Sky Pirates in the Ninjago range. There is also General Kozu, but his set is even more expensive.


Misfortune's Keep
The Lighthouse Siege
Samurai X Cave Chaos


As Kevin points out in the comments, Lord Garmadon appears three times in the LEGO Ninjago Minifigures series (Garmadon, Volcano Garmadon (pyjamas), and Flashback Garmadon (suit and tie)), so it's possible the part is now more widely available.

The key part from Nadakhan's body you need is:


  Torso, Modified Short with Armour Breastplate...


Or there are some other options from earlier Ninjago sets available as well.

Which accepts a standard neck post and has holes for the arms.