At more than 8 cents per gigabyte, archival DVDs are horribly expensive. You could cycle your backups across three hard drives for about the same amount of money, and then you have three backups instead of one.
Not to mention... have you ever tried backing up your 4 TB hard drive onto a spindle of 1,000 DVDs? Have you ever seen a spindle of 1,000 DVDs? It's slightly taller than an average person. Yes, if you don't have much data, you can do what you're proposing, but....
Hard drives are really the only viable backup medium unless you have a big enough collection of data for tape drives to make sense—maybe Blu-Ray, but only if you don't have more than about a 100-disc spindle worth of data (2.5 or 5 TB) to back up (and really, most people lose interest at more like ten or fifteen discs).