The Five Caesars would not include Augustus because he died of natural causes. And they would not include the Year of Four Emperors because they were short lived (except for Vespasian). But Vespasian and Titus also died of natural causes. So who are the five? Well there are only 5 left:
Julius Caesar, Tiberius, Caligula, Claudius and Nero. They all died either by murder or suicide.
http://en.wikipedia.org/wiki/List_of_Roman_emperors
