Upgrading RAM with 2 different memory modules
DDR memory must be 'matched pairs' to get optimum performance.  That being said, any performance increase (which depends on the kinds of tasks you're running) from that pairing effect would be vastly outpaced by increasing your RAM from 4 to 10 GB, thus minimizing swaps to disk.  I would not worry about it especially on such an old machine.
More technical info here