Damaged pool, water leaked out?
Heat of rods more intense than expected, water boiled away?
In desparation, they rerouted/siphoned the water to another reactor when it was perceived to be a bigger problem?
“Damaged pool, water leaked out?
Heat of rods more intense than expected, water boiled away?”
Probably both. The Pool at reactor 4 might have been damaged from the explosion in Reactor 3. And the pool of reactor 4 contained hot fuel due to the maintenance of reactor 4.