The four volunteers repeatedly set fire to hay bales or stores – or were aware of plans to do so.
In her ruling, the judge at the youth court in Ahrensburg, Schleswig-Holstein explained that without the leadership of one of the men, the crimes would never have come to fruition.
This ringleader was handed an 18-month suspended sentence for arson in a group and incitement to cause property damage, as well as 100 hours of community service.
His three fellow accused who confessed to abetting arson and property damage received fines of between €300 and €1,000.
All four of the former firefighters, aged 20 to 22, will now face compulsory psychological interviews.