...That is, there is a chance to kill 4 at once, which was not possible with just one herd of cows!

Everything depends on whether there is a limit on the maximum number of units that can take down a stack of Mighty Gorgons... The maximum number is equal to

D(max)=N/10+1, where N is the number of Gorgons in the stack (rounded down to the nearest whole number).

The most effective division in the example above would be: 1+1+1+1+1+1+16.
The first stack takes a retaliatory strike, the rest attack without losses. The maximum possible (additional) damage in this case would be 8 enemy units in the first round of battle, 6 in the second, 4 in the third, and 2 in the fourth... In total, 20 units in the end (if you're very lucky, of course :) )... But what if the opponent had Azure Dragons? :)