These titles refer to four Roman Emperors. History records that two of them were great and wise, and two were cruel and self-indulgent.
Trajan was Roman Emperor from 98 to 117 AD, best known for his extensive public building program which reshaped the city of Rome. He annexed several countries and expanded the Roman Empire to its greatest territorial extent. Every new emperor after him was honored by the Senate with the wish felicior Augusto, melior Traiano ("luckier than Augustus and better than Trajan").
Antoninus Pius was Emperor from 138 to 161 AD. He built temples, theaters, and mausoleums, promoted the arts and sciences, and bestowed honours and financial rewards upon the teachers of rhetoric and philosophy. His reign was the most peaceful during the first period of the Roman Empire (27BC to 284 AD).
Nero was Emperor from 54 to 68 AD. He focused his attention on diplomacy, trade, and enhancing the cultural life of the Empire. But his rule is often associated with tyranny and extravagance. He ordered many executions, including his mother’s, and possibly his stepbrothers, and had Christians captured and killed in various imaginative ways. He famously ‘fiddled’ while most of Rome was destroyed in a great fire, which many Romans believed Nero himself had started in order to clear land for his planned palace complex. In 68, rebellions in Gaul and Hispania drove him from the throne. Facing assassination, he committed suicide.
Caligula was Emperor from 37 to 41 AD. He is described as a noble and moderate ruler during the first two years of his rule. After this, he seems to have descended into cruelty, extravagance, and sexual perversity. During his brief reign, he worked to increase the unconstrained personal power of the emperor. He was assassinated in 41 AD, as part of a conspiracy involving officers of the Praetorian Guard and members of the Roman Senate.