CrystalDiskMark
It just so happens that seven other drives feature a 1TB capacity, compared to the 4TB here. Their size is still plenty fast enough for them to reach solid sequential and 4K random speeds.
Focusing on Corsair's claims, the MP400 is close enough to its sequential throughput specifications and represents a good match for an upper mainstream PCIe 3.0 drive such as the Kingston KC2500.
There's nothing to complain about on the 4K tests, either, so it's a good start.