If the backups were decentralized, all four servers could be restored simultaneously. Redundancy is also very important. Tape drives and servers are mechanical objects, and, as such, have a finite lifetime. When designing a large backup scheme with many servers and tape drives, it is important to plan to have enough unused tape space to cover the failure of any backup component. Tape media security is another thing that many people forget.
Backup tapes contain a complete image of a business' production data.
A used backup tape should never be casually thrown away. No industrial spy ever restored a backup tape that had been minced by a large axe. Media longevity is another point. Any backup tape rotation should include a provision for rotating out old tapes and replacing them with new ones. Tapes simply wear out over time.