CP has one of the worst tracking systems in the world, no joke. How were the tracking updates for the other 3 packages? Same as the missing one or did it have proper consistent updates along the way? There is a good chance that this 4th box was not scanned when picked up/dropped off. Best that you notify the sender asap, inform them that you have already received 3 of the 4, have them open a trace on this 4th box. After that, you yourself call CP and tell them the same thing, see if they are able to get a trace going for you. For the most part they only want to deal with the shipper, but it doesn't hurt to try, sometimes they will keep the receiver in the loop.