I suspect the market isn't there yet for 4 TB SSD drives... and it wouldn't require a 3.5" drive case, you could fit 4 TB of NAND easily in a 2.5" drive case (or even less).
Well right now, the 1TB enterprise quality SSDs have dropped below $900 (5 year warranty, super-caps, etc.). They're quickly edging out the 15k RPM SAS drives.
Consider that if I need X IOPS and a TB of capacity, I can either put 2x1TB SSD into a server and spend about $1800-ish. Or I can buy a more expensive RAID controller and try to put together half a dozen to a full dozen 15k RPM SAS drives. Using SSD means less drives, less power, less heat, smaller server footprint - same or better IOPS (usually 10x better).
The magical price point for enterprise storage drives is about $300-$600 per drive. Business IT won't blink at spending $300-$600 on a single drive, especially if it reduces their spindle count and increases performance. Something in the $1500-$2000 range tends to be a rarer purchase.