If there is a type 4 civilization I think we would know that...if there would be a type 3 in the milky way we would also know. There could be still thousands of civilizations of type 2 or lower, but I don't think the Earth or the solar system are interesting for them. There are enough planets out there. If they have the technology to come here and do so, they probably would because of humans.
I think transmutation of elements is another potential game changer. It could lower the cost of precious metals and rare earth metals, open them up for wider use in electronics. And it is something, most people would think it doesn't happen anytime soon. But I think hopefully we see that until 2020.
Edited by tornado64, 25 May 2012 - 03:55 PM.