The slower RPM should encourage stability (which has been notoriously destructive on larger drives) and lower power consumption. Generally speaking, large drives are meant solely to be storage drives -- not performance drives. Of course, if you wanted to buy one of these, our hardware experts would recommend running a mirrored RAID configuration in the event of a catastrophic failure. You really don't want to lose 4TB worth of data.
Hitachi's Deskstar 5K (model #HDS5C4040ALE630) has been spotted in Japan and has not yet been seen on US retailer online locations (please comment below if you spot it!). Judging by the ~¥27,000 price-point, we'd expect the drive to rank somewhere in the $300 to $400 range in the US. Quite expensive. The sad thing? Artificially-inflated drive prices due to flooding in Thailand make this drive a much better value than the $100 500GB drives you see out there currently. Things will improve soon enough.
Image Source: Akiba (translated from Japanese)