Two people were killed in each aircraft, which came down in woods near Waddesdon Estate, near Aylesbury, according to Thames Valley Police said.
Police chief Rebecca Mears said she could not yet give out the identities of the victims.
“Our priorities today remain with investigating the next of kin, finding out who they are, informing them and supporting them with specialist officers as we progress the investigation here on site,” she said in a statement.
“We anticipate being here until about Monday morning, potentially longer, we do not rush these things, it’s really important we do a meticulous investigation and really thorough to get to the bottom of what’s happened here.”
Both aircraft had taken off from nearby Wycombe Air Park, a spokesman said.
The Air Accidents Investigation Branch (AAIB) said it was assisting in the investigation, revealing that the plane involved was a light Cessna.
South Central Ambulance Service said the alarm was raised at 12.09pm (1209 GMT) local time “to a mid-air collision involving a helicopter and an aircraft.”
“We sent a number of resources to the scene, including the Thames Valley air ambulance, two ambulance crews, two ambulance officers and a rapid response vehicle,” said a spokeswoman.
Local gardener, Len Bellis told the Press Association that he found the “burning wreckage” after earlier hearing a “horrendous noise”, describing the light aircraft remains as “non-existent”.