I cannot speak offhand for the accuracy of the time period. I do know, however, that when new fish are added to a fully cycled tank, it may take a little time for the bacteria to "catch up" to the higher levels of ammonia. This is sometimes called a mini-cycle.
Zebra Danios have a rather low bio-load. The number of fish you can/should add will depend a lot on that. The more bio-load you add to a cycled tank, the higher the ammonia spike will be and the more time you'll need to wait before your biological filter grows to handle the new load.
If you're fully cycled, and we're talking Danios, then I would say you could get away with adding the other four for the sake of having them in a school. Just be sure to continue to monitor your ammonia, nitrite and nitrate levels. I say this as I bought two Danios once, to do the same thing, and had to get rid of one because it was attacking the other--more than just fin nipping. Found out the hard way that they tend to do that if not properly schooled. If your Danios are doing okay and there's no sign of aggression so far, then I recommend playing it safe and adding two more, wait the two weeks (provided there is minimal aggression), then getting the last two. But you could probably get away with adding all four.
