How to restore backed up contacts to my iPhone 4?
If you have sync'd your iPhone very recently, but just before the disappearance of the contacts, then you should be ok.  The first part of each iTunes sync process is a backup.

I would make sure the Address Book data in the Mac is backed up. I would then connect the iPhone, open iTunes, and sync.

If still no joy, then restore (an option in iTunes) the iPhone from an earlier backup to get them back, and then update the apps as needed.

If your iPhone is jail-broken or otherwise hacked, don't try the above until you have proven beyond a doubt that no haerm will be done.