My battery drains so fast

Not my experience with the 16 GB iPhone 4s and iOS 9.3.5. Your answer is in your settings, or in how you use your phone.

I went through every iOS application that requires location services, and switched it from Always to either Never, or While Using the App. The only application that I keep running in the background with Background App Refresh is Skype, all others are removed. I do not use Facebook or Twitter on the iPhone, and I restrict messaging. I do not use my iPhone for music or video playback. I do not use FaceTime audio or video on the iPhone. My Bluetooth radio is off. Wi-Fi is on all day.

Some very worthwhile reading.

When to charge your iPhone or iPad

http://www.macissues.com/2015/10/17/troubleshoot-and-manage-high-battery-usage-i n-ios-9/

http://www.macissues.com/2015/01/05/eight-battery-saving-settings-for-your-iphon e-and-ipad/#more-2767

http://solutionowl.com/the-ultimate-guide-to-solving-iphone-ipad-and-ios-battery -drain/

Settings -> Battery -> and scroll down to the Battery Usage. You can see what apps are taking up how how much of your battery in the last 24 hours, and in the last 7 days.