Political tensions between Bahrain and Qatar rose as Manama accuses Doha of illegally seizing 15 boats from the kingdom with 20 fishermen on board.
In a statement on its website, Bahrain's interior ministry said Qatar had seized three Bahraini boats with 16 aboard in the past two days.
Coast Guard commander Ala Siyadi said in the statement that this took to 15 the overall number of boats seized and 20 the number of fishermen.
The Bahraini authorities did not specify when the other four people were detained, but the ministry said some boats had been seized in 2009.
Meanwhile, Qatar accused Bahrain on Tuesday of abusing a routine fisheries enforcement case.
"The State of Qatar condemned the statement issued by the Bahraini interior ministry on the detention of 15 Bahraini boats carrying 20 sailors, describing it as a desperate attempt to escalate the ongoing diplomatic dispute," the official QNA news agency reported.
The official told QNA the Coast and Borders Security Department had repeatedly warned the fishermen not to operate in Qatari waters and not to use harmful fishing practices.
Qatar had a longstanding territorial dispute with Bahrain over the waters and small islands that separate the peninsula from the main islands of its maritime neighbour which was only resolved by the International Court of Justice in 2001.
In June this year, Bahrain joined with Saudi Arabia, the United Arab Emirates and Egypt in breaking off diplomatic relations with Qatar and severing all sea and air links as they accused Doha of supporting terrorism. Doha denies these accusations.
Qatar's emir, Sheikh Tamim bin Hamad Al-Thani, has held talks in Turkey, Germany and France over the past week to try to find a way out of the crisis and is due to meet US President Donald Trump in New York later on Tuesday.