I don't have a gs4, but have cached with plenty of other phones and I must say, you could absolutely use it as your primary GPS with the added benefit of having all the cache info right in your hand! AWESOME! That being said, The ONLY thing you have to worry about is how tough the outdoors can be on your phone. Rain, humidity,branches knocking it out of your hand and/or scratching the hell out of it etc. Give the official app a go! Even for the price, it is connected directly to tthe geocaching site and works rather flawlessly!
Anybody done it? Can I get rid of my dedicated GPS for geocaching?