RAID 4 is very similar to RAID 3, the only difference is that data is divided into smaller blocks (16, 32, 64 and 128kB). Performance has been improved by stripping data across many discs in blocks, also fault tolerance has been provided by dedicated parity disc. Comparing to RAID 3 and RAID 5, RAID 4 is mostly like RAID 3, the only difference is that uses blocks instead of byte for streeping, and like RAID 5 except that uses dedicated parity instead of distributed parity. Random access performance has been improved due to using blocks instead of byte, any way parity disc remains bottleneck, especially for random write performance.
Controller Requirements: medium to high end hardware RAID card
Hard Disk Requirements: minimum 3 hard drives, maximum is set by controller, all the discs should be the same size.
Array Capacity: size of the smallest drive*(number of drives -1)
Storage Efficiency: (number of drives -1)/number of drives
Fault tolerance: good, loss of one drive can be tolerated
Availability: very good, most of controllers support automatic rebuild and hot sparing
Degradation and rebuilding: rebuilds can take many hours, relatively little degrading of performance in case of disc fail
Random read performance: very good
Random write performance: poor, bottleneck of the parity disc and parity calculations
Sequential read performance: very good
Sequential write performance: fair good