What do the sheep bleat(instead of “four legs good, two legs bad” when the pigs enter the yard walking on their hind legs. Explain the significance of the modification of the original motto

I think you are referring to,

But just at that moment, as though at a signal, all the sheep burst out into a tremendous bleating of-

"Four legs good, two legs better! Four legs good, two legs better! Four legs good, two legs better!"

It went on for five minutes without stopping. And by the time the sheep had quieted down, the chance to utter any protest had passed, for the pigs had marched back into the farmhouse.

The original intent of the chant has been reversed. It now favors a human condition that the pigs have now adopted. The irony is almost palpable.