One of the four is in life-threatening condition and was being transferred to the bigger regional hospital in the northern city of Lille, prosecutors said.
The nearly two-hour fight broke out on the southern outskirts of Calais among migrants who had been queueing for food handouts.
A second fight then broke out at an industrial site around five kilometres (three miles away), with more than a hundred Eritreans fighting about 20 Afghans, prosecutors said.
"Police intervened to protect the Afghan migrants faced with 150 to 200 Eritrean migrants," the local prefecture said.
The notorious Jungle camp in Calais, once home to some 10,000 people hoping to make it to Britain, was demolished in 2016, but hundreds of migrants remain in the port city seeking to stow away on England-bound trucks.
Those left, most of them young Africans and Afghan men, have been living rough in the woods and clash regularly with police, who clear their encampments and stop them from setting up roadblocks in a bid to slow passing trucks.
Grim living conditions have led to regular confrontations between migrants of different nationalities, and five people were shot in a fight between rival Afghan groups last November.
Charities working with migrants in the area say around 800 are currently living in Calais, while local authorities put the numbers at 550 to 600.
Last month President Emmanuel Macron vowed zero tolerance for camps like the "Jungle" and secured a new border security deal which will see Britain pay more to stop migrants trying to reach its shores.
Macron has said he wants to step up expulsions of economic migrants while speeding up waiting times for asylum applications -- an approach he touts as mixing "humanity" and "efficiency".
But his tougher line has earned criticism from some of his allies, with his former senior aide Jean Pisano-Ferry among those signing a hard-hitting open letter claiming Macron risked betraying his image as a humanist.
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