We're not exactly reaching four billion people any time soon Yuu.
Yes, because by the time we reach four billion, the planet's rock dry surface would have magically turned to water by then.
The point is, we should only develop this planet to a point where our water-recycling capabilities can still support the population using the local resources. Recycling CO
2 and waste water can only get you so far when the colony's population eventually expands, even if it does take whoever knows how long.
Think of it this way: The number of consumers (humans) will steadily increase while the number of products (H
2O O
2) will not. Eventually, what you will get is a massive number of people trying to divide the few resources that are left. Sounds familiar? It was most likely the same reason why the colony was made in the first place, because despite how good people were at recycling the resources on the Blue Marble, the population eventually outweighed the resources necessary to sustain it.
[1]Recycling =/= Ex Nihilo
Now, we can say "Why not just do the same and send out our own colonies, or mine nearby asteroids, in case there's few water here?" The problem with that is that we're still not sure when we'll be able to make a functioning space program.
Now, if we were to find a large enough amount of H
2O on the planet, or if for some reason, these future people are actually capable of transmuting rock into water using a fission-fusion process, then by all means spread and multiply.

But until then, let's keep the population at the medium to bare minimum level required for running the preliminary operations.

In any case, the four billion thing was mostly sarcasm anyway. Several million would, judging by what the landers look like, be enough to make our current water supply insufficient for our needs.
[1]Note the term "most likely." Not a 100% precise conclusion, as only Hydro has the final say.
Ps. Thanks for the explanation regarding tornadoes. It was enlightening.
