Can I use my sim card in an iPhone 4?

So my cousin has an iPhone 4 that she offered to give me and I was wondering if I got it factory unlocked (since I'm not on AT&T), what would I do about the sim card situation since iPhone 4 phones use the micro sim cards? Is it possible for me to use this phone?

It may not operate on the frequency you need (the US uses several)
If it does have the right frequency, search cnet.com for a how to video that shows how to chop down a full size sim to a micro.

You can get the phone factory unlocked and trim your SIM to micro SIM and use it in your iPhone. Check online for factory unlocking iPhone.

Yes if you unlock your Apple I-phone 4 from network lock, you can use your sim card in the iphone 4.To get unlock it from network lock, use remote unlocking service. Visit the site http://www.mobile-unlocker.com/apple-iphone-4/rs2wp9/ get the remote unlocking service and unlock it easily. After unlock it you can use with any GSM network sim card. I-phone 4 support Micro-Sim card. If your sim is not a micro sim you can call your service provider and request micro sim card.