Wimberley P-50 Lens Plate
The P-50 lens plate is compatible with any Arca-type quick release plate. There are a few lenses which require a reducing bushing. These include the EF 400mm f/2.8 IS/ 600 f/4 (with and without IS). The P-50 has front and back safety stops to prevent the the lens from accidentally slipping out of the quick release plate.
This is the plate for your 400mm lens. Australia's ONLY authorised Wimberley Tripod stockist.