I would feel obligated to let them know they messed up and they would have to foot the bill if they wanted the stuff back. I am not a big fan of keeping something that is not mine. If they offered to let me keep one extra and only return 3 then that would be cool.
I found, after a little research, the proper thing to do in this situation, is to inform the company of the mistake. After 30 days of notice, if they do not contact you to reclaim the mis-shipped goods, they are yours. They cannot try to charge or reclaim the item after that.
That'* why I have two 37" LCD TVs at home instead of one.
Seeing as NewEgg figured it out and contacted the recipients, they should return them. They don't have to offer the $50, but it'* nice of them.