Nothing wrong with the ER-4, but let me break a lance for the ER-X.
It's true, it cannot do more than 1Gbps total (transmit + receive combined) due to hardware limitations, but given the asymmetric nature of most network sessions, that may not be an issue at all. It's certainly capable of filling a gigabit pipe both sending and receiving, just not both at the same time.
If you expect lots of concurrent large uploads and downloads, get the ER4. If you're a more 'standard' internet user (lots of downloads, not so many uploads), the ER-X may well fit the bill just fine. It certainly does for me (I got the ER-X-SFP because my provider allows and even encourages tech-savvy people to use their own hardware, and the SFP port allows me to directly connect to their incoming fiber)