4xhdd test speed with smart array e200 controller

[Arvudas]

Hi,
Have HP Proliant ML 110 server and HP smart array E200 controller. When i use one hdd and test it with HD tune program, he show me average 50MB/s speed. That is one hdd speed, normal. When use 2xHDD in RAID 0 array this program show me average 100MB/s. when use 3xHDD show me 150MB/s speed, but when use 4xhdd in RAID 0 array, then this program show me only 175MB/s. Should be average 200MB/s. Server, controller, driver, hdd is always same. Where can be problem?