I got this magnetic fisheye lens from photojo online. i had to do a little modification on the metal “O” ring that you need to attach to the phone so your lens can magnetically connect. I used my hand dremel to cut the ring so the flash would not be covered. I have a iphone 3G not a 4G. The supplied rings come with an adhesive back. I removed it and just used clear silicone to attach the ring to my phone. That seemed to do the trick. The lens cost around $20 and is a great and inexpensive way to add a fisheye to your iphone. They had some others as well including a wide angle and a telephoto.
fisheye for iphone