In 48 BC, the Roman Republic was decaying rapidly. Caesar had crossed the Rubicon River in the year before, starting a civil war. Pompey, Cato the younger and the rest of the conservative faction of the senate had fled to Greece. Caesar was preparing to follow in pursuit and, among other preparations, levied Legio IV Macedonica. The first battles of the legion were Dyrrhachium and Pharsalus, where Caesar defeat Pompey. After this, the legion was stationed in Macedonia province, attaining thus its cognomen.
IV Macedonica sided always with Julius Caesar adopted son, Octavianus, first against Caesar murderers in the battles of Phillipi in 42 BC, then against Marcus Antonius in the naval battle of Actium in 31 BC.
Octavianus, now Augustus, sent the legion to Hispania Tarraconensis in 30 BC, to take part in the campaign against the Cantabrians. After Augustus victory in 13 BC, the legion remained in the province, but its effectives were spread through the Iberian Peninsula.
In 43AD, the legion was transferred to Germania Superior, to replace Legio XIV Gemina as the garrison of Moguntiacum (modern Mainz). Along with XXII Primigeneia, the legion supported Vitellius, governor of Germania Superior, in the year of the four emperors (69 AD) first against Otho, then Vespasian who would become emperor.
During the Batavian rebellion (69/70 AD), IV Macedonica secured Mainz and fought under Potillius Cerealis against the rebels. Their actions deserved no reproach but Vespasian did not trust its men, probably due to their support for Vitellius. The legion was disbanded in 70, but reconstituted shortly afterwards under the name of Legio IV Flavia Firma.
See also: List of Roman legions, Roman legion