How does Snowball show his ruthlessness?
Are you sure you mean Snowball and not Napoleon? I'm sure that I would agree with Snowball being ruthless except when dealing with Mr. Jones during the first attempted re-capture of the farm. Napoleon, on the other hand, could be described as ruthless in several instances throughout the novel, starting with his vicious removal of Snowball in chapter five.
Snowball's plans to destroy Mr. Jones and the other men by cutting off their escape route is the example you're looking for in chapter four. Having studied the attack methods of Caesar, Snowball wasn't interested in simply turning back the attackers; they needed to be finished, which was what the animals attempted to do.