Has your iphone 4 or 4S slowed dramatically since you upgraded to iOS 7? Has your battery life noticeably declined? Here is an article on how to restore your old phone to iOS 6.
Make sure you back up your iphone before you perform the downgrade!
If you have not yet updated to iOS 7 and your phone is an older model with slower hardware or a weaker battery, you may wish to consider not updating to iOS 7.