Proper size is (2*width) x Height, so in the Nexus 4's case, a properly sized wallpaper will be 1536x1280 (width x height). The visibility of the navigation bar and status bar don't affect the proportions of the wallpaper - the extra bits of wallpaper are just hidden behind the bars.
If you don't want the wallpaper to scroll and you just want a static 768x1280 image, you can use an app like Image 2 Wallpaper
to set that, and just disable wallpaper scrolling in your homescreen launcher.
Edit: Actually, I just did some testing. The wallpaper behavior depends on your homescreen launcher. So with Nova Launcher (what I use) it disregards the navbar when setting wallpaper, and 1536x1280 correct. BUT
, if you use the stock Launcher, then it does take off the navigation bar dimensions. That's why the stock wallpapers are 1436x1196 - they work differently than 3rd party launchers. So pretty much: use 1536x1280 for Nova Launcher (and probably Apex too), and 1436x1196 if you use stock launcher.
LG Nexus 4 - 8GB