Can I put my iPhone 4 sim card into a new iPhone 4S?
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'.