2 quick questions on my RN104 OS6 screens

The formula for single redundancy is to sum the total of the all the disks, and then subtract the largest. This is in TB (multiples of 1000), not TiB (multiples of 1024). The NAS (like windows) reports TiB.

First hot-insert the 4 TB into slot 4, (e.g, remove the bad disk, and insert the new replacement with the NAS running). Then wait for the drive to be synced (added into the array). Then after that is complete, hot-insert the second 4TB drive into any of the other slots. If all the disks are solid, perhaps pick the one with the most power-on hours (which you can see in the SMART stats).

What about the "raw read" and "seek" error rates? Either way slot #1 has the most hours and the highest numbers for the raw read & seek error rates. So I can replace the disks in slot#1 & #4 and have the expansion work fine? Good to hear. I'll replace #4 first, wait for it to resync (a day or so until it states it's done).
And the replace the other disk in slot #1.

Also, since the S.M.A.R.T. table above shows a "normal" temperature, what's up with the temp shown an the performance tab?

seek error rate and raw error rate have vendor-specific formats, which makes it difficult to know what to make of them.

I don't know what's going on with the performance tab data, but since you are replacing the disk anyway I don't think its a concern. If you still see the -1 after the new disk is inserted and synced, then submit a support request on that at support.netgear.com.