Depends on your situation, though in general I recommend moving up to the Drobo Pro (vertical upgrade) rather than adding another Drobo.
Once you pass the second additional chassis, you’ll lose space on each additional, though I guess with an 8-bay Drobo you’d be even, though the risk is distributed differently.
In addition to the things Docchris mentioned, you also would have almost the same amount of protected storage (just slightly less) with Dual Disk Redundancy, or more protected storage if you use Single Disk Redundancy.
That said, I have 4 Drobos, but my storage configuration is quite atypical (and arguably overkill)
I have two Drive Bender pools:
Drobo v2 + Drobo v2
I use this for my movie storage - I don’t really need the protection, so it’s mainly a matter of convenience.
This functions like a simple spanned volume.
Drobo S Gen 1 + Drobo S Gen 2
This is my “normal” shared storage. Both Drobo S units have Dual Disk Redundancy enabled, and Drive Bender is doing software mirroring between the two units for specific folders.
This functions like a RAID 6 setup with selective RAID 6+1. This, of course, is in addition to normal backups to a separate portable drive.
Again… I am NOT typical (and depending on who you ask, perhaps just plain crazy)
Drobo provides fault-tolerance. Fault-tolerance is not a substitute for backups. Fault-tolerance simply reduces the downtime and (hopefully) small data loss between the time of last backup and time of failure.
For some this is simply a convenience, for others this is well worth cost of investment.