You came from a Two-Drive RADI 1, add two more drives same capacity. Now the system recognizes that these drives would enable the RAID 5 configuration. Therefore the system kindly asks you if you want to have all drives working together in a RAID 5. This is called RADI Migration, from a lower RAID level to a higher RAID level.
Of course, you are right. During the RAID-rework, a power cut would be deadly. Therefore, a backup of the data would be a good choice prior to the RAID migration.
No. Creating another array on the newly inserted drives will NOT affect the exisiting array on the other drives.
No. The formating process is the last step in the RAID-Create process. First, the drives will be packed together in a RAID array, then - within this RAID array - the file system will be created (aka the partition will be formated and the file system will be created). This “job” cannot be done outside the system as the RAID array has to be created by the system itself.
You answered by yourself Follow the on-screen instructions (carefull reading of the on-screen texts may be helpfull)
Just joking; yes, it is pretty simple - but as usual, don’t hurry and keep calm.