Hi.
A change is as good as a rest so they say !
With the amount of idevices you have it does sound like your in Apples Eco System
You should do a little homework on how to add your contacts to software such as GMail for example.
Also on what to do with any music in your iTunes. It's all available on you tube and via google on the various ways of moving data over.
The only thing I will say about the S4 is the way up to half of the reported internal capacity is taken up with the specs it has ,and that some of it is partitioned off for future updates so 16gb drops to around 8gb.
Apart from that it's an awesome device.
If you haven't used it or android yet is suggest popping down to one of the high street shops to have a play around with one before you make the jump into changing.
I've just recently started field testing phones for O2 which to date has had the Android Os ,and found the change pretty easy and didn't have to adapt too much.
Enjoy the journey and perhaps update your thread on your choice.