1. make sure you have 1 empty sata port
2. after you attach your 1 10TB to your old build, make it spare
3. shutdown system and disconnect of your 4TB
4. power up machine
5. your synology tried to repair the lost disk.
6. repeat this step until all your 3 10TB are occupied. now you have 1 x 4TB and 3 10TB in your system