Richard,
• 4 drives are pretty unlikely to all die at precisely the same time (as long at they are in separate enclosures), but it sounds like you only have 2 copies of any one file. I strongly suggest 3. - primary, online backup version, and off-line/off-site backup version. Outlined in the book. (Have you read it?)
• You say 2 sets for archiving. I strongly suggest that the primary archived version is on hard disk, and that the first backup also be hard disk (and taken off-site). The Optical disk backup is the disaster recovery version.
• Yes, the 15 packs seem to the largest packages I see at any reasonable price. Give them a try. I also noticed that SuperMediaStore has the same deal.
• Memorex has been around for decades. Not the highest reputation, but not terrible either. They don't make disks themselves, at least that's what the wikipedia entry indicates. In any case, the proof is in the pudding, and, as I say, all my memorex disks are validating with 100% accuracy. If I find any problems I'll post here.
http://en.wikipedia.org/wiki/MemorexPeter