Guys, after doing lots of research I found that, there's nothing wrong with any of your apps, nothing wrong with your usage or the way you charge your phone. The fact is Samsung released a huge bunch Faulty/Defective (Faulty battery with poorly built handset body) Galaxy S4. The problem is widespread and more critical than you think. Please read the following article:
Samsung Galaxy S4 battery problem hitting users ? Phone Reviews
Here goes the exact problem details (Copied from my previous post): I bought Samsung Galaxy S4 (GT-I9500, Made in Vietnam) from Kogan Australia in August then I left Australia and going to live overseas for a while. For last 4 days I am experiencing really strange battery draining issue. My phones battery drains down to 0% (mentioning battery critically low) and turns off by itself. This happened almost everyday for last few days even when my phone battery had over 85% or 60% or 40% charge left. After took the battery out of the phone and then re-inserted and re-booted, it still shows battery critically low. Then I did a factory reset on my device and I am still experiencing the same problem. It is also important to note that, yesterday after the phone turned off by itself by showing battery critically low (in reality it had over 85% charge left), I left it off for a while without plugging it in for charge. Then I turned it on again after a while, and the the phone rebooted fine and was working fine with almost 70% charge.
Now the most Dangerous part! After around 2/3 months usage the Battery will experience Swelling all of a sudden and in the process will bend your Handset Screen outwards (Uneven screen raised outwards from the handset). This happens because the battery swells and looks like a flat pillow and can not fit inside and as a result bends your expensive handset. Samsung should start a product recall immediately and replace all the handsets!!!!