You would have to look up that info on this website by searching international plans and you can put in where your going it will tell you the fees for doing calls text data etc because the 4 is not a global phone.
Thank you for your reply. I have in fact searched the calling rates for using a CDMA iPhone 4 in Japan. Japan is a CDMA country so I am confident that I will be able to make and receive calls at the rates posted on this website once I enable international roaming.
My uncertainty lies in whether I will be able to send text messages from the iPhone 4 while in Japan. I am not sure if the iPhone 4's SMS sending protocols will work with Japanese systems. I have read that cellular phones may require a different UTMS frequency band for using SMS in Japan.
If you have any other information regarding this issue that would greatly appreciated.
Thank you again.