Don't forget, that the spent fuel in the pools contains some plutonium, and there is a lot of fuel rods in those pools -- many more than in the reactor core. And there is virtually NO containment around them. The hydrogen explosions have blown holes in the building -- this is where they are going to attempt to shoot water with the water canon/fire truck. This is a pretty desperate move -- what else can they try? It is too radioactive to fly over with a helicopter...
The last two that we have not heard about -- reactor #5 and #6 also have spent rod pools, that may be less hot than #4, but they may well evaporate their water soon, and make matters much, much worse.