Not necessarily. You said originally your 4TB was nearly full. That in itself will drive significant file relocation and parity checks. As others have stated... chill and wait it out. No biggie.Presumably, this means that each time I add a disk to the array it will take longer than the previous time to rebuild because there will be more data to calculate?
... and yes, delve into the 🍪 stash I left for others.