Noel Chong, head of the International Maritime Bureau (IMB) Piracy Reporting Centre, said the recent attacks took place very near to each other, but he was not able to say whether the same group was responsible.
"Whether it's a different group of pirates, we can't tell until an investigation is carried out. We have sent out an urgent warning to all ships travelling through the Gulf of Aden," he said.
On Tuesday, a Malaysian oil tanker carrying 39 crew members has been hijacked by pirates in the Gulf of Aden, between Somalia and Yemen, a maritime watchdog group said.
MISC Berhad, the company which owns the seized tanker 'Bunga Melati Dua', said the ship was hijacked at 1409 GMT.
The four ships had a total of 96 crew on board, including nine on the German vessel.
"We have already informed the US-led coalition naval forces who have dispatched a warship to intercept the tanker believed to be heading towards Somalian waters," he said.
Pirates usually demand a ransom from the owners, he added.
"This is the fourth ship being hijacked in a month in the waters," Chong said.
"We call upon the UN and the international community to take immediate steps to stop the worsening security situation in these waters," he said.
A statement, issued by MISC Berhad, said Bunga Melati Dua was heading towards the Dutch port of Rotterdam from Dumai, Sumatra Indonesia at the time of the incident.
"The ship has 39 crew, of which 29 are Malaysians and 10 Filipinos," it said.
The waters off Nigeria and Somalia are the world's most dangerous hotspots with 24 attacks reported in Somalia and 18 in Nigeria between April and June, according to IMB figures.
Of the 24 Somali attacks, 19 occurred in the Gulf of Aden off the country's north coast.
Last week, a Thai cargo ship was hijacked off the Somali coast by pirates from Somalia.
Last week, pirates from Somalia hijacked two ships - a Thai cargo ship and a Nigerian boat.
Somali pirates are still holding a Japanese-managed vessel, the MV Stella Maris, that was hijacked on July 20.
In most cases, Somali pirates have treated captives with care in the hope of receiving substantial ransom payments.
Naval forces from the United States, France, Germany, Pakistan, Britain and Canada are operating in the Gulf region.