All about risk and how important your data is.
If you do RAID 5 with one spare, I would imagine when a drive fails, it immediately spins up the spare disk and starts returning the array to a redundant state. You can then remove the failed disk, and bung in another one ready for the same event to happen again.
If you skip the spare, then there will be a period where the array is in a non-redundant state until you replace the failed disk and it rebuilds it again.
Swings and roundabouts. Personally, I would do a RAID 5 setup on a 4 drive NAS. If you had 8 drives, then RAID 6 is another option (you can lose two drives) or even RAID 6 + spare, but with only 4 drives, I would skip the spare option.