mdadm: Replacing array with entirely new drives

Create a whole new array with the new drives, it's not worth the risk or hassle to mess with old drives. Consider carefully the type of raid you are setting up. RAID5 on drives that large leads to very long rebuild times during which you are vulnerable to a second drive failure and complete data loss. There are strategies to reduce this risk, such as using RAID6 where you can survive losing two of the drives.

Also consider running LVM on top of your new raid array as this will give you options for growing or migrating filesystems in the future.

If your goal is to convert a 3-device mdadm RAID5 array into a 4-device array, I believe your only choice right now is your first option: Create a second array with the four 1TB drives, then copy all your data over from the first array.

As far as I know, the mdadm "grow" mode does not currently support adding more devices to an existing RAID5, so, unfortunately, you can't use your second option. Or rather, if you do go with the second option, you will end up with a 3-device RAID5 on 1TB drives, with a 1TB drive left over as a spare. (I believe you could configure that extra drive as a hot spare.)