Joseph Brown, 36, of California, Md., and William Johnson, 37, were arrested in Raleigh, N.C., The Washington Post reported Tuesday. Quinita Ennis, 30, described as Brown's girlfriend, and Edwin Jones, 40, of Lexington Park, Md., were picked up in Lexington Park.
Ennis and Jones were ordered held without bail after an arraignment Monday.
Investigators said $110,000 was found hidden in two safes buried in Brown's yard.
The PNC Bank branch in St. Mary's County was robbed Sept. 24. Brown and Johnson allegedly kidnapped the bank manager, LaToya Booth, and her young son and held the toddler hostage while she removed money from the bank.
Initially, investigators suspected Booth was involved but now say the robbery was done completely from outside. They charge that Brown and Jones planned the job, and the group watched the bank for several weeks, studying traffic patterns.
They allegedly spent much of the missing money in Atlantic City and on iPods, computers, clothes and plane tickets.
"All the co-conspirators basically went on a shopping spree," said Lt. Rick Burris of the St. Mary's Bureau of Investigation.
Hometowns for Johnson and Ennis were not listed.