Personally, I would keep the drive as a hot spare. The server at my school has a hot spare that was inactive for 8 years and only recently needed to come back up so there is practically no downside to having a hot spare vs a cold one but make sure that the hot spare is setup to automatically start rebuilding data in case of a drive failure.
Using Raid 5 would be a good idea basically no other config needed.
12 minutes ago, PCoverclock said:
put your most important stuff on that thing an store it away.
OP. I would suggest getting an offsite backup if the data is important and use the 4th drive as a hot spare.