I think the new Nexus 7 is evidence enough that Google has decided to stick with the brands Nexus 4, Nexus 7, and Nexus 10. It's their way of marketing them as a Nexus family. Sundar also stated the Nexus 10 would be updated later in the year and it will still be called the Nexus 10. It's no different then how Apple labels their iPads, iPods, and computers; or how car models are named. You stick with one name so it can become a known brand, and then just upgrade it with years or generations. Leave it to the other Android OEMs to confuse consumers by changing names with each new flagship.
It's called a Nexus 7 because there was no screen size change because they didn't feel a need to change the screen size. Same thing with the Nexus 10. They are both tablets and are at good sizes.
The Nexus 4 is a little different. 5" phones are becoming the new flagship size. Just look at GS4 sales numbers.
It's not a matter of keeping the brand. They have a brand. It's called Nexus. You see a Nexus and a number and you know exactly what it is. 4 doesn't mean smartphone, it relates to the screen size. Expect a 5 inch screen on the next Nexus. Expect a Nexus 5.