The fourth century BC legionnaire wasn’t quite yet the stereotypical soldier of late Republican and Imperial times. Some major differences:
- From phalanx to cohort: Marcus Furius Camillus had the roman phalanx evolve into the more flexible cohort that would give so much trouble to Phyrrus in southern Italy, but this change would happen only after a couple of decades into the fourth century (somebody has a more precise estimate?).
- The pilum rather than the gladius: the famous gladius (the famous Roman short sword) would be adapted from the Hiberian Celts only later. In the fourth century, the short spear (pilum) would be the main hand-to-hand weapon. Thus the legionnaire would still fight very much as a hoplite, using the spear above or besides his large shield to strike at the enemies’ face, throat, chest or legs. Most legionnaires would throw javelins while closing in, and use a sword when the pilum is gone (into an enemy, bent, broken or otherwise). He was protected by a bronze or polished iron helmet, a cuirass (lorica) and the shield.
- No marriage: girlfriend(s) for sure, but no marriage for legionnaires. Can’t have the head of the family killed on the battlefield (might be other reasons?). That goes for the whole century, as the requirement of celibacy would be lifted by Ceverus. Same-day update: Nephele at UNRV points me to this discussion thread. Some sources there I might have to check out if the opportunity presents itself.
- Not ‘professional’ soldiers: they would typically live from their land, get paid only for time the spent on campaign, an amount that would vary. Campaign pay was cause for considerable upheaval within Roman society, especially when legionnaires back from long campaigns would find themselves deeply into debt upon their return, their land having been left uncultivated, or worked by hired help. At some point their equipment started being paid for by the state (instead of being the soldier’s responsibility), not sure when.
- Latin: Roman integration of populations beyond Latium as citizens was accomplished only starting in the second half of the century (better check this statement for accuracy but it sounds about right). Thus the Roman military would still be massively Latin citizens who don’t stray far from home. Under the Empire, legionnaires would be from a wide range of cultures and extremely well-travelled.
So we are still talking about a military that is built along largely traditional Greek lines: armoured, landowning citizens fighting behind a spear wall.
About about legion organization in the fourth century BC: Roman Army at the Illustrated History of the Roman Empire.
Legion history and equipment: this old Geocities page isn’t bad at all.
Further research: got to be clearer about when and how standard legion equipment started being handed out by the state. Possibly only when the land ownership requirement was dropped (in the 2nd Century B.C.?).