You should be able to add the 4TB drives to the Synology DS418 and then add the 12TB drive as expanded storage as long as they're compatible with the DS418. Here is a Synology Knowledge Base article on the process:
Add a Drive to Expand Storage Capacity
As far as the ramifications to the storage, that depends on how you're laying out the storage. Here is the
Synology RAID Calculator Tool which can come in extremely handy in determining storage layouts in a NAS or RAID array.