Alright, so you wanna know why your battery is draining rapidly and some questions about the charger, right?
Alright, here are your possible answers.
1) The S6 should not drain battery that much in Power Saving mode. Ensure that your phone is not in heavy use and make sure that all apps are closed and Wi-Fi, Bluetooth, NFC, mobile data and other connectivity features are off when not in use. Check the battery graph in the Settings app to get a general idea of what's draining your battery.
2) Yes, you can use your iPhone 4's charger or even the iPad's charger to charge the S6. It won't damage your phone* (at least in my experience) but the only difference is charging speed. The iPhone 4's charger is 1A and the iPad's charger is 2.1A. Both will charge slower than the included Adaptive Fast Charger, which can charge at up to 3A.
You don't have to ask what's the suited voltage. The phone's charging circuitry and circuitry in the charger itself will determine that for you, so all you need to do is just plug it in.
* - I still recommend using any Samsung charger just for peace-of-mind.
NOTE: I recommend using the iPhone 4's charger for nightly charges since it draws less power and thus, doesn't give the battery much heat compared to the iPad and included S6 charger. Only use the S6's included charger when you really need some more battery power in a short period of time.