My HTC One X recently dropped and the screen shattered so I began looking on eBay for a used iPhone 4. Is switching to the iPhone a big hassle? I already have the micro SIM card that the iPhone 4 takes.
No, switching to an iPhone 4 should not be too much of a hassle for you. You should call customer service and let them know that you need to change your data plan from a regular smartphone data plan to an iPhone-specific data plan. Yes, it kind of seems silly to do so, but it needs to be done so that you can use your iPhone to its full potential.
If your microSIM card is still active, simply pop it into the iPhone once you receive it and you should be good to go.