By assigning photos to contacts individually you will see the persons photo on screen while they call you. So, just follow the steps written below to assign photo to individual contact on iPhone 4.
Assign picture to a contact in iPhone 4
Tap on “Contacts” on your iPhone 4 home screen and tap the Contact from list to which you want to assign photo.
You will see the “Info” screen for that specific contact, so tap on “Edit” button at right upper corner of screen.
On upper left corner tap on “add photo”. It will give two options as “Take Photo” and “Choose Photo”. We tap “Choose Photo” to select photo from photo gallery on iPhone 4, you can choose “Take Photo” option if you want to take photo on the spot.
Photo Albums will get open, so navigate to your required photo and tap on that for full screen view and tap “Choose” button.
Photo will be added to contact now tap “Done” on right upper corner of screen.
Now, when you will receive a call from that specific contact you will see the assigned photo on your iPhone screen. In the same way you may add photo to your favorite or all of your contacts one by one on iPhone 4 when you have some free time.