It doesn't quite work like that.
1) When you have more than 2 disks in the Drobo, it doesn't just create a mirror/backup of your data. It is helpful to think of it similarly to RAID5. Basically, each chunk of data is split amongst 3 of your 4 drives. Drobo "does math" and calculates a parity bit for each chunk and stores this on the 4th drive. Every time you store data, the "4th drive" changes.
So, there is no actual backup disk. The parity information is split evenly amongst all the disks.
2) There is no way to disable BeyondRaid. The Drobo doesn't present the individual disks to the OS. As far as your Mac is concerned, you just have a single Very Large external hard disk connected.
3) You are mostly right, if one of the 4 disks fails, the green light next to it will turn red. You just pop it out, and put a new one in. You can do this while it is running even continue using the Drobo while it rebuilds.
Unfortunately, it is not possible to setup the drives in a D1, D1A, D2, D2A scheme. That would be similar to RAID1/0 and Drobo functions similarly to RAID5. One of great things about Drobo is that it is Simple. It just works, you plug in drives, and it manages them however it thinks is best. This does lead to some fairly inflexible configurations. There are other NAS devices that perform the same functions as Drobo but allow greater flexibility. But, I found them to either be more expensive or require a bit more time to setup/configure.
I'm sure you've seen their BeyondRaid description page, but if not: http://drobo.com/resources/beyondraid.php