They were arrested in multiple raids across the capital on Monday night, said Rajib Al Masood, deputy commissioner of the police’s detective branch.
The arrestees are Hannan alias Robin alias Rafiqul Islam, 50, Md Mostafa, 52, Md Babul Mia, 55, and Parveen, 31.
Police have recovered Tk 6m and two firearms from them, said Rajib adding an operation is underway to retrieve the rest of the money.
On May 10, an executive from National Bank's headquarters along with two armed guards picked up money from its branches in Old Dhaka, according to Kotwali Police OC Mizanur Rahman.
They were en route to the main branch in Motijheel when one of the security men yelled out about a missing bag of cash as they reached Babubazar.
An official of the National Bank subsequently filed a case with the local police station and the men were detained. However, police were unable to extract any clue from them.
The law enforcers later apprehended the criminals by analysing footages from CCTV cameras.