Thinking seriously, I do not think I would be able to predict much apart from a great urge to get married for the sake of itself.
This would be due to the fact that largely people would be unpredictable with such a prognosis as "4 years before Earth's destruction". If Astronomical evidence could prove that this would be indeed the case, many large nations would shift funds towards extraplanetary colonization operations, obviously. But aside from that, it's anyone's game if society would still retain value in money and the world would operate as usual for a little while, or quickly devolve into anarchy or worse with the justification consisting of only "It won't matter in 4 years".
This would be due to the fact that largely people would be unpredictable with such a prognosis as "4 years before Earth's destruction". If Astronomical evidence could prove that this would be indeed the case, many large nations would shift funds towards extraplanetary colonization operations, obviously. But aside from that, it's anyone's game if society would still retain value in money and the world would operate as usual for a little while, or quickly devolve into anarchy or worse with the justification consisting of only "It won't matter in 4 years".