(cffi:defcstruct vec4-t
  (value :float :count 4)) ; place for 4 single floats  

The graphene:vec4-t structure is capable of holding a vector with four dimensions x, y, z and w. Use the graphene:vec4-x, graphene:vec4-y, graphene:vec4-z, and graphene:vec4-w functions to get the values of the components. There are no functions to set the values directly.