Which iPhone 4 cannot be unlocked?

[Denis54]

I want to buy an iPhone 4 and would like to know which ones cannot be unlocked.

Is there any baseband version that cannot be unlocked?

Is there any iOS version that prevent unlocking?

 

[takeshi74]

The CDMA ones can't be unlocked because it's not relevant to them.

 

[mattopotamus]

you either need to buy and unlocked phone (GSM) or have one of the GSM carries unlock the phone for you (ATT/T-mobile). I'm not sure, but if you are under contract you may not be able to have your phone unlocked since technically you could get an iphone for cheap, unlock the phone, and cancel your contract for even cheaper than out right buying a new iphone. If they let you do this, they are stupid

 

[Bugeyeblue]

The Verizon iPhone 4s can be unlocked to work with international gsm carriers and will roam using at&t's gsm towers here in the US. Here's a video, it's not mine, but I have done this with my Verizon iPhone 4s.
http://www.youtube.com/watch?v=KPDv8S-Izrg&feature=youtube_gdata_player

Edit... Sorry, just noticed you said 4not 4s.