Iphone 4 and iphone 5 SIM card switch?

I currently have a iPhone 4 and I'm thinking of buying a unlocked iPhone 5 on eBay, if I were to do that would I be about to use my SIM card in the iPhone 5?_. I don't know if this is a stupid question or not! Haha sorry of it is ^^;

Its not stupid, you just don't know.
you can't use any other iphone sim card because iphone 5 uses nano sim cards so yea and other iphones use regular sim

Yes you would, but make note the 5 uses nanoSIMs vs the microSIMs of the 4.

Transfer number to a new phone