Mail is common among Celtic nobles and beginning to appear more widely. You're right that mass-produced mail didn't appear til later in the Roman era
In the 4th BC century mail is a relatively new invention though. Early history is a bit clouded and uncertain, many ascribe it to Celts, but some to Etruskan. They appear quite frequently in Chieftains graves in the 3rd century, but I haven't seen any from the 4th century cases. Mostly they are found in the eastern part of the 'Celtic'-area (Romania and Bulgaria and further east to Turkey).

To my knowledge the earliest find of mail is still the weapon deposit at Hjortsping on Als C14dated to 350calBC (so in the 4th century). How many were found is uncertain as preservation was terrible and they were excavated in 1920'ies so they are now lost. The original author suggest 10-12 or so, but that is likely very exagerated (its based on the area where rings were found, but they were scattered).

A discussion can be found here.

Though that ignores that the earliest is found in the northern area, and that it is likely that the lack of mail to the north and west could be due to lack of evidence (no pictoral evidence and different grave contexts). The Celts in the east were "foreign" overlords or invaders, and thus where more heavily militarised than those in the west.

So for a 'classic' (Greek etc) PC mail could be made 'exotic' and great loot (if the GM so wished).

It should also be noted that there is very little to separate 'Celtic' and 'Germanian' material culture in the 4th century BC (there are more differences within each group). These really develop later (around the 1st century BC).