1 Dead drive, how do I recover the data on the 3 good drives?

I'm running FreeNAS 9.1.1 with 4 1TB drives in a single ZFS volume. I believe it's a RAID 0 setup, because I have all 4 TB available. Now, one of the drives is essentially kaput. It's the last of the 4, by chance. I can't boot into FreeNAS now because during boot, after trying to mount the local file systems, I get scrolling ATA errors, ending with these:

ada3:ata5:0:0:0): READ_DMA. ACB: c8 00 90 02 40 40 00 00 00 00 10 00
ada3:ata5:0:0:0): CAM Status: ATA Status Error
ada3:ata5:0:0:0):...

1 Dead drive, how do I recover the data on the 3 good drives?