I know this topic is stale but I wanted to share how I got my MSS working. I too had problems getting all 4 drives recognised.
I have a video breakout board and was able to go into the BIOS and check the SiS interface chip config. I found that by changing the mode from 4P(IDE)+4S(RAID) to 4P(IDE)+4S(IDE) all the disks were found.
I have now been using the unRAID for several weeks (2x2TB data + 1TB cache + 2T parity) and am generally pleased. The only problem I have is that the 1GB/s NIC is only working in 100Mb/s mode. I suspect this is a unRAID driver issue. Any ideas?