code: func writeTo4Header(w io.Writer, id uint8, data []byte) error {
  	if len(data) > 0 {
  		if err := binary.Write(w, binary.LittleEndian, id); err != nil {
  			return err
  		}
  		if err := binary.Write(w, binary.LittleEndian, uint32(len(data))); err != nil {
  			return err
  		}
  		if err := binary.Write(w, binary.LittleEndian, data); err != nil {
  			return err
  		}
  	}
  	return nil
  }
docs: // writeTo4Header is an helper to write a file header with the correct Kdbx v4 structure to the given io.Writer