I need a really large battery Android phone. It should be able to charge other devices using it's own battery power. It must run a relatively new version of Android (4.3 or newer). It must also cost less than $400 unlocked. Other specs are not important but the higher end the better.
Well, considering the fact that you need to charge other phones the phone must have a huge battery. The new Gionee Marathon M5 will definitely suit your needs with a humongous capacity of 6020mAh. The phone runs Android Lollipop out of the box and boasts an AMOLED display. It comes as a surprise that Gionee has managed to fit a huge battery relative to current standards in an 8.5 mm thick shell (consider it a bonus). It also has an OTG(On the go) cable for charging other devices. There is also another device manufacturer named Oukitel (Chinese) which made the Oukitel K10000. I do not recommend this because it has a relatively low performance and a bad display.
Ulefone Power also has a ~6000mAh battery and can charge other phones/devices and is priced very reasonably.
Update: I will retract this as a recommendation as I have some concerns about the quality/reliability of mine.
Lenovo P2 has ~5000mAh, which can last me over a weekend at a pinch.
This gives it slightly less ability to charge other things than the 6000mAh suggestions, although with the P2 itself appearing to be very power efficient, needing less power itself, the difference might not be so big.