Depends on your needs. If you need/want a 4TB partition then there's only a few ways to get one:
1) A single drive with 4TB capacity. Speeds are that of a single drive and reliability is that of a single drive. With no redundancy, it's an all-or-nothing situation: either the drive works or it doesn't.
2) A pair of 4TB drives in RAID1. While it doesn't help the speed any, you now need both drives to fail in order to lose any data.
3) A pair of 2TB drives in RAID0. Increases speed at the cost of increasing the risk of failure. Since it offers no redundancy, if either drive fails then you lose everything -- that means the data on the good drive is useless.
4) Three 2TB drives in RAID5. This offers 4TB of usable capacity since you lose one disk's capacity to redundancy (the parity is split across the drives unlike RAID3 where a dedicated drive stores parity information). Reads are faster since it's combined from 3 drives, and writes can be faster or slower. Faster since they're split across three drives - so a 3GB file being written is three 1GB chunks written in parallel. Slower because it needs to calculate parity so it actually writes 4.5GB. That is, this looks like a 4TB partition and the file is 3GB, but on the disk level it's a 6TB pool and 4.5GB was written.
5) Four 1TB drives in RAID0. While possibly faster than the 2x2TB RAID0, the risk of failure is essentially 4 times as high. If you experience a drive failure every 3 years, then you'd expect to lose everything about every 9 months.
6) Four 2TB drives in RAID10. The speed of RAID0, the redundancy of RAID1. You can sustain one drive loss and possibly two before you lose any data.
8) Four 2TB drives in RAID6. More parity calculations than RAID5, you lose the capacity of two disks so this is 4TB usable. Since the parity is distributed, you can lose any two drives before you lose data so this is more redundant than RAID10 though not as fast.
7) Five 1TB drives in RAID5. Similar to three 2TB drives, except now you need 5 disks. Since 5 > 3, the odds of a drive failure is higher but since RAID5 can sustain one drive loss this should be OK. You'd be more likely to encounter needing to replace a disk simply because you'd have more of them, but if you can get a replacement in and the array rebuilt before another drive fails then it's not a problem. A hot spare can help here, but that means six 1TB drives and you could do something else with that...
9) Six 1TB drives in RAID6. It's probably faster than four 2TB drives in RAID6, but this is starting to use a lot of drives so you'll start to run into problems (a) connecting them and (b) housing them.
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