The Nexus 4 will not get the GE Launcher that the Nexus 5 has, at least not to begin with. We might see it as an update in the Play Store down the road, but
for now Google has confirmed it's exclusive to the N5. However, that doesn't mean a whole lot as far as the transparency thing goes. As a mod pointed out in another thread, the transparency feature is simply a software call available to any app that requests it. Third party launchers like Nova Launcher are already playing with this in beta versions, so you will likely see that feature pop up very soon in the public release versions.
This does signal a departure from the norm for the Nexus program in my eyes, though. The Nexus 5 is no longer the "pure Android phone." It's now Google's phone, which is a subtle but distinct difference. We can no longer look at the latest Nexus device, running the newest version of Android, and think, "Wow, I can't wait to get Feature X on my older Nexus device!" From now on, we'll have to look at the feature list of the next version of Android and
hunt for the asterisks indicating exclusivity.