You can only hold 4 cargo on your ship. You can take the 4 new cards add them with the 3 you had, but you'll have to dump what you cant hold overboard. If the new 4 cards are better, dump your old cargo for the new!
As far as I understand it. You don't have to choose between your first cargo or the new one from your event/rumor/mission/conquest. You take everything in your hand and chose which cards you dump and which ones you keep.
Maybe that is what was meant with the earlier answer, though :-)