The WD RE4-GP matches the desktop Caviar Green's capacity but increases buffer size to 64MB, adds in a five-year waranty (three on Caviar Green), quotes a 1.2-million hours before MTBF, and time-limited error recovery - a RAID-specific feature.
Additionally, WD reckons the drive is better-suited to the enterprise space because it employs what's termed a rotary acceleration feed forward feature, whereby drive is less prone to failure in vibration-heavy environments - in multi-drive rack-mounts, for example.
There's no explicit mention of spindle speed, just like the desktop Green, so it's safe to assume it's a variable-speed drive, probably spinning closer to 5,400rpm at most times.
Priced at $329.99 it is $30 more expensive than the desktop variant. That should make it around £275 in the UK.
Specification link.