Can I still use my iPhone 4 if I get a iPhone 5?

Hi, I'm thinking of getting a iPhone 5 but I don't want to trade in my iPhone 4 and 100 bucks to get a iPhone 5 and instead buy the iPhone 5 at the 200 bucks price with a 2 year contract. If I was to get a iPhone 5 which gots new SIM cards can I use it which both iPhones or can't and should trade it in since its basically useless now. Also this is AT&T

Yes it will work but the 3g, calling, and texting won't work.