Sorry to hear that. Two reasons I can think of, that may cause the transformers to be damaged, are either surge / over-voltage, or there is a persistent shorting somewhere. The latter is probably more likely, given that you have 4 transformers damaged. One thing I suggest you to check is whether there may be a shorted or semi-shorted solenoid somewhere. You can use a multimeter to measure the resistance of each sprinkler solenoid (the simplest way, without taking down the wirings, is to measure the resistance from COM to each individual station wire). Normal 24V AC solenoids should have about 30 to 50 ohm resistance. If the resistance is too low (like below 10ohm), you should replace that solenoid.