Can I put my iPhone 4 sim card into a new iPhone 4S? I have an iPhone 4, and want to upgrade to an iPhone 4S. Can I just buy one and switch my sim card?

My current iPhone is through my ex's account. I don't want him knowing I'm getting a new one. My name isn't on the account, so I was hoping to just purchase the phone, then put my sim in there.
If you buy a new iPhone and switch SIM's, you'll be fine.

Both the iPhone 4 and the iPhone 4S are using the microsim, so SIM's will fit in both iPhones.

If your current iPhone 4 is registered on the account of your ex, he won't get a notice of SIM's being switched.
However, if you reconfigured the iPhone with a new account, your ex might know the phone isn't registered with his account when using apps like 'Find My iPhone'.