OpenCl array inside a struct

Hello,
I have been having problem sending a relatively simple structure to GPU:

public struct Structure
{
   /*some other Vectors, floats or integers*/
    public float[] Array;
 }

Corresponding Cl code:

struct __attribute__ ((packed)) Structure
{
    /*as before*/
    float4 Transform[4];
};

And while I have no problem with converting Vectors to float4, I have no idea how to proceed with fixed-size arrays.

GCHandle.Alloc(data, GCHandleType.Pinned);

throws an exception, saying that object has wrong data.

Any help?