"Florence has been described as a city with two opposite currents of life, one directed by the fervent and austere Savonarola, the other by the splendour-loving Lorenzo. Savonarola's influence upon the young Machiavelli must have been slight, for although at one time he wielded immense power over the fortunes of Florence, he only furnished Machiavelli with a subject of a gibe in "The Prince," where he is cited as an example of an unarmed prophet who came to a bad end."^1
Girolamo Savonarola was a Dominican friar who took to preaching fire and brimstone sermons about the coming apocalypse. A few of his predictions seemed to come true when Charles VIII of France invaded Italy, Savonarola stirred up a mob, drove Piero de Medici out of the city, allied himself with Charles, set up a theocratic republic with himself as the de facto dictator and began industriously purging Florence of people he deemed immoral. Which leads us to...
" It is necessary, therefore, if we desire to discuss this matter thoroughly, to inquire whether these innovators can rely on themselves or have to depend on others: that is to say, whether, to consummate their enterprise, have they to use prayers or can they use force? In the first instance they always succeed badly, and never compass anything; but when they can rely on themselves and use force, then they are rarely endangered. Hence it is that all armed prophets have conquered, and the unarmed ones have been destroyed. Besides the reasons mentioned, the nature of the people is variable, and whilst it is easy to persuade them, it is difficult to fix them in that persuasion. And thus it is necessary to take such measures that, when they believe no longer, it may be possible to make them believe by force.
If Moses, Cyrus, Theseus, and Romulus had been unarmed they could not have enforced their constitutions for long—as happened in our time to Fra Girolamo Savonarola, who was ruined with his new order of things immediately the multitude believed in him no longer, and he had no means of keeping steadfast those who believed or of making the unbelievers to believe. Therefore such as these have great difficulties in consummating their enterprise, for all their dangers are in the ascent, yet with ability they will overcome them; but when these are overcome, and those who envied them their success are exterminated, they will begin to be respected, and they will continue afterwards powerful, secure, honoured, and happy. "^2
Basically, those four are the armed prophets that Machiavelli refers to, in contrast to Savonarola. As Machiavelli notes, Florence got tired of the deranged friar after a while.