They were four young pigs throughout the book, that always protested against Napoleon's decisions. We never found out the names of the pigs. Then Napoleon had all four of them killed.
They are first seen protesting when Napoleon cancel the Sunday meetings, where the animals vote. They are quieted by the dogs. Then they are seen protesting again at another part of the book and again are quieted by the dogs. Then one days, Napoleon calls a meeting, the dogs grab the pigs and drag them to Napoleon, then Napoleon has them killed. Also during the execution, they confess to crimes of spying for Snowball. Please note that Napoleon most likely forced them to do so as Snowball was most likely deceased at the time.
They represent the numerous political figures whom were killed during Stalin's Purges.