Glad you brought up the chipset architecture - I hadn't really given thought to the degree of efficiency moving to 7nm architecture would provide. Turns out that, generally speaking, a 7nm chip is about 40% more efficient than 10nm. If that's accurate and were that the only difference in devices, the P4 could have come with a 2100mAh battery and come out about the same, or with its 2700mAh battery would be equivalent to what the P3 would achieve with a 3,780mAh battery.
Of course, that's just the processor - the screen at higher refresh will certainly eat into that surplus. And then there's all the new sensors. The question becomes, to what degree.
It's entirely possible that those who tend to frequently use apps that are much more taxing on the CPU may find that they get comparable or even better life on the 4 with the smaller battery. Of course, those who don't run CPU-intensive tasks on their 3 won't see as substantial an energy savings there, and so will almost certainly see diminished effective battery life.
Have to wait and see I suppose. Google may well have screwed up horribly with the 4, given that one of the chief complaints about the 3 was the battery life, but maybe - just maybe - they did ok.
Personally, I'm hoping that the 4 is fully on par with the 3 in terms of effective battery life, because if it is then the 4XL should have substantially better life than the 3XL which, for me, has nearly always been more than sufficient for a day.