The Camera Roll is for temporary storage. Photos/videos in the Camera Roll can and should be imported by your computer ASAP as with any other digital camera. The Camera Roll is also included with the iPhone's backup, which is updated by iTunes as the first step during the iTunes sync process.
If the photos/videos were not imported by your computer and your iPhone wasn't synced with iTunes since the photos/videos were captured by your iPhone, there is another way to
recover photos from iPhone 4.