A stash of drugs including heroin and cocaine were found hidden in secret compartments of a fake ambulance, police said.
Wraps of amphetamine were also discovered in specially made hiding places behind interior panels, in cupboards and under the floor.
The National Crime Agency said three holdalls containing thousands of what are believed to be ecstasy pills were also found.
The seizure of around 270 kilos of drugs followed a surveillance operation by NCA officers who watched as two of the men in paramedic uniforms arrived in Birmingham in the ambulance on Tuesday morning and met the other two, a spokesman said.
The officers, who were assisted by colleagues from West Midlands Police, moved in and arrested all four men at a car park on Hill Street in Smethwick.
Another two men from London seen leaving the car park shortly before the ambulance arrived were also arrested nearby.
Brent Lyon, from the NCA's armed operations unit, said: "This was an audacious plot. We believe the two men in the ambulance posed as paramedics to avoid unwanted attention when entering the country through Harwich. Our officers were ready and waiting though and stopped the drugs from being distributed to crime groups across the country."
Four Dutch men - Olof Schoon, 37, Leonardus Bijlsma, 55, Dennis Vogelaar, 28 and Richard Engelsbel, 50 - were charged with drug offences and appeared at Birmingham Magistrates Court today. All four have been remanded in custody until July 2 when they will appear at Birmingham Crown Court. The two men from London have been bailed pending further enquiries.
