It is possible that two disks have failed simultaneously but it is unlikely. I have been in a similar situation before. I suspect that you may have a firmware error in all four disks even though it is presently only manifesting itself in two of them.
Do you still have warranty support with Dell? If so, I would contact them for detailed hand-holding as sorting this out is both complex and also involves a fairly high risk of data loss as it is not guaranteed that the solution will fix the problem.
I would also get the system tag no and look up Dell support on-line for tech advisories in respect of disks. You may have to identify the disk manufacturer and model.
If you are suffering the same problem I did the annoying thing is that Dell classified the fix as non-urgent. The fix involves identifying the order in which the disks failed, attempting to restart them in the same order and then updating the firmware on the disks and finally updating the firmware on the disks that are still working. The devil is in the detail as you would expect, hence the suggestion to speak to Dell support.
If the fix fails then the only other solution is a full tape restore.....
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