You’ll want to backup your stuff before doing anything as either a Mirror or RAID set as the two drives are dependent on each other.
As far as setting up as odd set (one 4TB and one 8TB drive) you can’t really RAID them as you won’t get the added benefit of the extra 4TB the 8TB offers. You would need to replace both drives as a matched pair (two of the exact same make & model).
Now you could use the two drives independently, but then you loose either the mirror so if one drive dies you don’t loose your stuff, or if RAIDed then the performance of the striping the data across the drives.
Frankly, I only use 4 or 8 drives in a RAID set for my work and also mirror them to a second drive case. But my needs are somewhat unique.