legally activate iPhone 4 factory locked.

Depends on who it is locked to?

Call Apple Care in your country and they may tell you

http://support.apple.com/kb/index?page=servicefaq&geo=United_Kingdom&product=iph one

Find your country in the drop down menu and ths service contacts will be displayed

Based on your question, I'm assuming that the phone is locked to AT&T. Since ONLY the carrier can authorize unlocking the phone, you're out of luck on that score. AT&T will not unlock iPhones under any circumstances.

That said, there is a difference between "legal" and "supported". In the US, and I imagine in many other countries, it is not technically illegal to unlock a phone by other means. Doing so, however will void the warranty and any Apple care agreement as well as forfeit any right to support from Apple or in these forums.

You would be better served to sell the phone and purchase one you can use in your country.