I feel like the next Nexus will probably be named the Nexus 5 and Google will emphasize the number 5 in this release. It would also have a perfect setup for Google since they would have a Nexus 5 with a 5-inch screen, Nexus 7 with a 7-inch screen, and Nexus 10 with 10-inch screen.
The Nexus 4 was named 4 because it was the fourth Nexus phone, while it does't have exactly a 4-inch display. The other Nexus devices' names correspond directly to their screen size.
Google would be able to emphasize the number 5 by:
- Android 5.0
- 5-inch 1080p screen
- Released in May (fifth month) --> when the Google I/O event is
- The fifth Nexus phone (previous release was Nexus 4)
- It also allows Google to go directly against Apple with the iPhone 5.