The Only way that would happen is if you are using a messaging application from a 3rd party or the Android market that is unavailable in the AppStore, or incompatible with the iPhone.
Use the stock messaging platform and this should NOT happen.
It is happening though, to me too. As much as I didn't want to have to, it looks like I'll have to go back to the iPhone. Too bad things are never what they're said to be......
The issue is NOT the phone. The S4 like any other android device and text to any smartphone. Now unless your friend with the iPhone is using IMessage then that's on them.
maybe this has an answer