How to recover deleted photos iPhone 4

All photos transferred from your computer via the iTunes sync process - selected under the Photos tab for your iPhone sync preferences with iTunes should remain on your computer.
Photos transferred from your computer are stored in your iPhones Photo Library and are not included with your iPhones backup since there is no reason for it.
Photos in your iPhones Camera Roll are included with your iPhones backup. Although photos in the Camera Roll are included with your iPhones backup which is updated by iTunes as the first step during the iTunes sync process, photos in the Camera Roll can and should be imported by your computer as with any other digital camera.
 MacBook Pro 15" Intel Core 2 Duo/2.53GHz/4GB RAM/500GB HD Mac OS X (10.6.6)  iPod w/Video 30GB,  iPhone 4 32GB