Thunderbay 6?

Great question!
The easiest way to answer is doing some rough math.

RAID 5 with 4 drives means 2 in 4 drives have to fail to lose data.
RAID 5 with 6 drives means 2 in 6 drives have to fail to lose data.

However, what are the odds of losing two drives before a replacement can be purchased and installed?

Lets simplify this and do some rough math:

There is a 72 hour window, say, from a disk failure before a replacement can be installed.
Drives (already in service) fail at an average rate of about 2% per year.

(Note: I am using the established rate of failure, despite the fact that SoftRAID reduces this dramatically because of the "Predicted failure" feature, where it becomes much less likely that a disk can fail unannounced.)

Using the 2% figure, then on a single day, the odds of a drive failing are about .0055%
With a 3 day failure window, that is approximately .015% risk factor.
Lets compare 3 remaining drives to 5 remaining drives:
3 drives: .045%
5 drives: .075%

So the quick answer is yes, the odds increase but that goes from about one time in 50,000 to one time in 35,000.

These odds are not zero however, and there are many other reasons to back up, so maintain backups of all volumes, even RAID volumes. Do not lapse into false security always maintain regular scheduled backups!

In the years since we released a RAID 5 version of SoftRAID, I have not seen a scenario where two drives failed at the same time. I have seen two drives predicted to fail happen nearly at the same time, which of course has a much higher risk of two drive failures.