raid6 + hot spare question

["Peter Keše" <peter.kese@viidea.com>]

I'm planning to set up a raid6 array with 4 x 4TB drives.
Presumably that would result in 8TB of usable space + parity, which is 
about enough for my data (my data is currently 5TB in raid1, slowly 
growing at about 1 TB per year, but I often keep some additional backups 
if space permits).

However I'd like to be prepared for a disk failure. Because my server is 
not easily accessible and disk replacement times can be long, I'm 
considering the idea of making a 5-drive raid6, thus getting 12TB 
useable space + parity. In this case, the extra 4TB drive would serve as 
some sort of a hot spare.

My assumption is that if one hard drive fails before the volume is more 
than 8TB full, I can just rebalance and resize the volume from 12 TB 
back to 8 TB essentially going from 5-drive raid6 to 4-drive raid6).

Can anyone confirm my assumption? Can I indeed rebalance from 5-drive 
raid6 to 4-drive raid6 if the volume is not too big?

Thanks,
     Peter