Everything you said makes a lot of sense and in terms of stand by time your points are valid. However, all of that might be great for stand by times but for screen on times I'm not so sure. The biggest power usage of any phone is the screen. The Nexus 5 has a 5" 1920x1080 IPS LCD vs the Nexus 4's 4.7" 1280x768 IPS LCD. The Nexus 5's screen is about 10% bigger in total area which means it will take more power. The screen also has more than twice the number of pixels. That means more work for the GPU and therefore higher power usage. It would help a lot if the Nexus 5 had the same GRAM that the LG G2 had but based on what I saw in the leaked service manual I don't think it does.
I really want this phone so if there is something I am missing please correct me. I want my next phone to get ~4 hours of screen on time over a period of ~16 hours. From what I can tell that isn't going to happen with the Nexus 5.
Oh I agree with you regarding the screen, but in general I'd say we've gotten better SOT over the ages of smart phones, and I'm not planning on keeping my screen on for 10 hours a day. I just don't want it to drain to hell in my pocket like my Nexus 4 does.
From what I've seen battery life under "real world conditions" for the S4 are pretty bad. From 100% to shutting down in 9 hours with 6.5 hours of that time in stand by. I've never owned, used or monitored use of an iPhone for an extended period of time. Is the idle time really worse than the S4?
This is a really YMMV and I didn't really want to bring this into an iPhone versus Android conversation, but in general you need to consider how an iPhone works compared to an Android phone.
First of all, iOS uses push exclusively. iOS7 might add a bit more multitasking so you can have more background tasks, but if you're pushing the standard email, Facebook, etc, your battery won't drop much on iOS. On Android on the other hand, these apps sync. Gmail syncs. Google+ syncs. It's push and sync. Facebook and Twitter used to force you to sync in order to get push notifications.
Depending how you set up your Android phone, it can be one major battery nightmare, or it could be pretty decent. On the other hand iPhones are harder to drain to hell unless maybe you turn on Find my Friends or somehow an app gets locked in place using the GPS.
I've used quite a few Android phones and from what I've seen, iPhones run great in idle. My iPhone 5 is pretty good even on vacations where I take hundreds of photos and have Google+ sync using LTE.
You hear stories of people getting great battery life on Android, but typically those users stick to Wifi, or disable a lot of features here and there. For example, my friend told me this weekend that his Nexus 4 needs to charge 3 times for his gf's 1 charge on the Moto X. While that may be true, I'm pretty sure it's because she turns everything off and runs solely on Wifi. She has a mini 200mb plan and has to do everything to prevent running over that.
Anyway, it's a long discussion, but I have yet to see anyone run any idle/standby battery tests. Those synthetic benchmarks and rundown tests only tell you about screen and SoC efficiency. They don't really tell you how the phone behaves in idle. I may be putting too much emphasis on idle, but honestly, a lot of phone usage is idle for many people. It sits in their pocket until a notification comes in. If we're all limited to around 4-6 hours of screen on time, the the rest of the screen off time better be idle and not awake, and that's where the different devices start showing their strengths and weaknesses.
Coming back to the S4, if we base the battery life on GSM Arena's "endurance rating," it's pretty solid. It's twice as long as a Nexus 4 which makes me think its amazing. But who knows? Until we have some sort of controlled idle test (i.e. push 2 gmails every hour, 2 facebook notifications an hour, Google Now on, Google+ syncing, etc.) it's really hard to determine how phones really do in your pocket. There's just too many variables. Now with apps like Google Now, and other location based apps that continuously ping you for your location, I bet idle times also differ if you're moving around versus if you're stationary.