How to transfer SIM card to iPhone 4?

My dad just gave me his old iPhone 4, since he just got the new 5, and I'm wondering how I can reassign the phone to my current one's (LG Rumour 2) number. I know that the iPhone 4 has a slot for the SIM card, but somehow I doubt it's as simple as popping the iPhone one out and putting in my LG Rumour one. Basically, I'm asking how I can set this up as a new iPhone with my old number (contacts don't matter that much since I don't have a lot). Does anyone know how I can do this? Any help is much appreciated.

You'd have to get a microSIM. Go to your carrier & ask for one, of course with your number assigned to it.