Because a RAID configuration masses all the drives together into one large array; none of the individual drives can be 'explored' on their own, nor would you want to with a RAID 10 array. RAID 0 stripes the data between two alternate drives to increase write speeds; the stripes can be of varying width but I think the default is usually 64k. So two of your drives are working together as a 'super drive', doubling the capacity and increasing write speeds. RAID 1 then mirrors these two striped drives onto two more spare drives. In this set up, you get the speed of RAID 0 with the redundency protection of RAID 1. In theory. I am learning the hard way that any RAID setup is a fragile beast, and having a redundent array does not protect you from data loss and all valuable data should be backed up anyway.
How can you verify that all 4 drives are being used? Because Intel Storage Matrix reports the raid as 'okay'. The one 'C:\' drive you see should be the size of two of your drives added together; the other two drives are being used to back up your data. But back up valuable information elsewhere anyway.