RAID Storage Concept unleashed - Part 3:

RAID 4: Block-level Striped Disk Array with Dedicated Parity (using XOR)
• Data is Block-level striped, except that one drive is reserved to hold parity data.
• The parity is the result of a XOR operation performed on the Blocks of the striped data.
• The minimum number of drive required for RAID4 creation is 3 and maximum is no limit.
• The capacity of a RAID 4 group is (n-1)*s, where n is the number of drives, and s is the size of the smallest drive in the group.
• RAID 4 can withstand upto one drive failure and if more than one drive fails, the data in RAID4 array will be lost.
• Despite good features RAID4 has many drawbacks such as very low write performance, Difficult and inefficient data rebuild in the event of disk failure & so on.

RAID 5: Block-level Striped Disk Array with Distributed Parity (using XOR)
• Both Data & Parity are Block-level striped accross all the drives of array.
• The parity is the result of a XOR operation performed on the Blocks of the striped data.
• The minimum number of drive required for RAID5 creation is 3 and maximum is no limit.
• The capacity of a RAID 5 group is (n-1)*s, where n is the number of drives, and s is the size of the smallest drive in the group.
• RAID 5 can withstand upto one drive failure and if more than one drive fails, the data in RAID5 array will be lost.
• The performance of RAID 5 is very good, only slightly slower than RAID 0 on both writes and reads.

RAID 6: Block-level Striped Disk Array with Independent Distributed Dual Parity (using XOR & Reed-Solomon ECC)
• Like RAID5, Data & Parity is striped on a block level across a set of drives, and a second set of parity is calculated and written across all the drives;
• The actual dat storage capacity of a RAID 6 group is (n-2)*s, where n is the number of drives, and s is the size of the smallest drive in the group.
• The minimum number of drive required for RAID6 creation is 4.
• RAID 6 array can withstand upto Two drive failure & Two different algorithms are employed to achieve this purpose.

Note: Apart from basic RAID levels, there are extended raid levels such as RAID01 (0+1), RAID10 (1+0), RAID50 (5+0) and so on.
these are mainly results of combination of two or more basic raid levels to achive higer performance & Redundancy.
Extended RAID levels are also extensively implemented and used in the existing RAID products.
Detailed information about the same can be found in http://en.wikipedia.org/wiki/Nested_RAID_levels.