Photos are being swapped between both iphones
When you sent your iPhone 4 to your friend, the device was not properly wiped first. As a result, all of your accounts are most likely still signed in on his device.

The best recommendation is to completely erase the device. To do this instruct your friend to go to Settings > General > Reset > Erase All Content and Settings. This will wipe the phone and restore it to factory settings. Your friend can then proceed through setup and plug in his own accounts.

If your friend is not willing to wipe the entire device, I recommend he signs out of all of your accounts. He will want to go to all of the following places to do so:


Settings > iCloud
Settings > Mail, Contacts, Calendars
Settings > Messages
Settings > FaceTime
Settings > iTunes &amp; App Stores
Settings > Music


You may also have apps that were left installed on your iPhone with your account signed in as well (Facebook and Twitter, for example), that you may want to also check.