Nigerian authorities have charged two Germans, an American and a Nigerian for breaching the official secrets act.
Dr Judith Burdin Asuni has lived in the region for years
The German journalists are accused of endangering national security by taking photos and video footage of oil facilities in the oil-rich Niger Delta.
The US-born aid worker and a Nigerian man are accused of helping them to make false declarations to obtain visas.
None of the accused entered a plea to the charges. If convicted, the Germans could face up to 14 years in jail.
Germans Florian Alexander Opitz and Andy Lehmann had been released by the Nigerian State Security Service (SSS) on Tuesday after being held for two weeks on suspicion of espionage.
But they, along with the US-born aid worker who facilitated their visit, Judith Burdin Asuni, and Nigerian man Danjuma Saidu are being held by the SSS until their next court appearance on Friday.
Prosecutors accused the four of acts considered "prejudicial to the security" of Nigeria.
Mr Opitz and Mr Lehmann had been filming masked youths from the Ijaw community in Delta State in September.
Delta militants have been conducting a violent campaign for a larger share of oil revenues.
At the time of her arrest the US embassy in Abuja told the BBC that it was in touch with the Nigerian government over the continued detention of Mrs Asuni, who is married to a Nigerian and has lived in the region for 36 years.
"All we know is that Judith Asuni is a peace worker who got funding from academics and international donor agencies to work for peace in Nigeria," the embassy said.
Mrs Asuni runs a high-profile non-governmental organisation called Academic Associates Peace Work and has run workshops with the Nigerian police on conflict management.
The German nationals were reportedly conducting preliminary research for a possible TV documentary about the Niger Delta.
The BBC's Alex Last says the Niger Delta has been a sore subject for Nigerian authorities, particularly the international attention given to militant groups, but it was rare for authorities to charge any accused "spies".
The region is home to all of Nigeria's oil, responsible for 95% of hard currency earnings, but most of the peoples of the Delta live in abject poverty.
The main militant group in the region, the Movement for the Emancipation of the Niger Delta, recently called off a truce with the Nigerian government after the arrest of one of its leaders in Angola.
And with a new president in charge, civil society groups and journalists are waiting to see if this is an isolated case or whether authorities will exert tighter control in the troubled region, our correspondent says.