With this type of a discount, its clear Google is attempting to clear inventory before the release of a successor (which we expect sometime in October or November). If you don't need 4G (LTE) speed then this is a fantastic deal for an unlocked phone.
Comparing the Nexus 4 to the Nexus 5
Based on the rumors I am hearing, the next version of a Nexus phone may have a bigger higher resolution screen but the quality difference will be more of a technical win than a practical one. The Nexus 4's screen is so good any upgrade wouldn't be noticeable to average Joe.
We expect a slight bump in processor speed but considering the Nexus 4 has a quad-core processor, improvements will be incremental and not revolutionary.
We expect the Nexus 5 to have significantly improved GPU performance for better gaming performance. Which could be good but maybe not enough to justify changing a phone.
We expect the Nexus 5 to have LTE. If you need LTE then the NExus 5 may be better but for most users, good 3G performance is perfectly acceptable.
If you need a new unlocked phone then this is a fantastic deal for an unlocked phone.I say get it while its available.