Why did reactor's 1, 2 and 3 continue to run at the Chernobyl power plant after the disaster at reactor 4.

I've been reading about the Chernobyl disaster and I recently learned that even after the disaster at the reactor 4 the other three reactors remained up and running for several years after (Reactor 2 was shut down after a major fire in 91, reactor 1 was shutdown in 96 and three in 2000). Does anyone know why they kept them running?

That is a good answer. Even in other countries, all the big generators are needed at peak demand time = very little reserve, so likely there were rolling black outs just with number four producing no useful power. A million people would have been without electricity at peak demand time, most days, if they shut down all four reactors. Evacuation helped solve that problem with fewer nearby users of electricity. Likely many of the workers who stayed behind to keep the other 3 reactors running now have cancer, or are dead already. Worse the the same reactor design was used in near by cities, so it would have been logical to shut these down also. Part of the problem was human error = the disaster might have been avoided, if the employees and supervisors had been smarter and more focused. Likely human error was part of the cause of later shut down of the other reactors. Neil

You know, that is an excellent question. I have a theory on the matter, but it's somewhat unlikely, and very unwise if it is the case:
Were the nuclear power generators the only sources of power for the city of Chernobyl? This is, in my opinion, quite unlikely, but one nuclear plant could create a lot of power. Is it possible that there were no other generators of any kind throughout the city? Yes. Is it likely or smart? No, and HELL no, respectively. But, I still recognize it's a possibility. After all, authority figures have made bigger mistakes.