Hi,
double4 is an hidden type that one can use in the setAttr function.
It works as excepted when one use it like
setAttr "node.attribute" -type "double4" 0 1 2 3;
But it is not present in the documentation though it is used internally in Maya to manipulate quaternions.
As an example, double4 type are used in GLSL when one declares a uniform vec4 variable and needs to configure it via MEL script or python. I encountered personally this need when I created a GLSL node in the VRay plugin with a vec4 uniform variable.
To be clear, my suggestion is not to add support for a double4 type into MEL or python, it is already here.
It is just to add this type to the documentation so it can be used by customers.
