When they say that it replaces 4 drives they don't count the capacity but the I/O only. It does happens that you end up with many drives just to get the right I/O number but only need a fraction of the space.
I don't consider this to be a real SAS drive. The performance will never macht, the access times are simply way to high compared to a 10k or 15k sas drive. However it could be a good solution for those archive solutions where you don't allways need that much I/O's.