Factory unlock iPhone 4
Your logic is not correct. For a while, there were people offering unlocking services for around US$170 who are believed to have bribed an employee of Apple or a mobile phone operator to unlock phones using the official channels, and those phones also didn't need to be jailbroken. There's no way I know of to distinguish one of those phones from a factory unlocked one.

Assuming it is a genuine factory unlock, there should be no problems -- I have a UK factory unlocked iPhone 4 and have upgraded the OS several times, though I've never tried to do so in another country.