I can't believe nobody has told you this, but the biggest reason your iPhone 4 had better battery life is because it is a 3G phone. Since your S4 is 4G, it will use more battery due to radio switching and the sheer power demands of the 4G environment! That is the nature of the beast. That extra inch of screen size will have a big effect as well.
I'm a light to moderate user and usually have at least 60% left by day's end! So, as was stated before, buy yourself a spare battery and swap out if/when you run low (can't do that, Apple!).
Fire up that S4 and don't look back!