Question on delegate Func<CustType, string, string, byte[]>

The VS2012 (.NET 4.5) compiler complains the following code:

public Func<CustType, String, String, byte[]> TokenServiceImpl { get; set; }

SomeType GetToken(CustType c, String server, string app, out byte[] key)
{
    if (TokenServiceImpl == null) {throw new exception("xxxxx");
   
    // This is the line where it gets compiler error
    return  TokenServiceImpl (c, server, app, out key);
}

Error:  Delegate "Func" does not take 4 arguments.

I have spend quite some on it but still cannot figure it out.
Can someone offer any suggestion? Big thanks in advance.