if you use 4T harddisk, then RAID5 is too risky. The rebuild time of large harddisk will make RAID5 protection non-existant. RAID5 is only reasonable if you use 2T harddisk or less. 4T needs RAID6 to make sense.
Also, do not buy the same model harddisk from the same manufacturer at the same time, they are more likely to fail together because they were built in the same batch.