Here's my simple question: if I have 4 hard disks all in RAID 0, they will work twice faster, than if I would have 2 hard disks all in RAID 0? Thanks.
Yeah that's true. With RAID 0, the data is divided into the amount of hard disks, giving each disk only a portion of the data. So every hard disk has less jobs to perform and has therefore higher performance. The more hard disks you use in RAID 0, the less jobs each disk has, thus increasing the performance. But there is a downside with RAID 0. When 1 hard disk fails, then all data is gone.
