According to Russian website Almodi.org that managed to snag some screenshots of WD's plans for the next year, it appears that Western Digital wants to bring 5TB drives in both its Green ant the Red Series in Q4 2013.
In addition to the 5TB WD50EFRX Red series and the WD50EZRX in the Green series, the Q3 2013 will also bring 4TB drives in both series. Of course, we are talking about 3.5-inch drives that will feature 64MB of cache and SATA 6Gbps interface.
The slides also do not reveal any info regarding standard 3.5-inch Blue series and 2.5-inch Scorpio line of drives. As you may remember, WD has recently announced a 4TB version in its Black series lineup so 5TB one might come sooner than in the Green or WD Red NAS line.