With the exception of the thinner profile, it really is the spitting image of the Z3. And the thinner profile isn't necessarily a reason to celebrate, because with that thinner frame comes a smaller, less powerful battery. In all fairness though, the Z3 offers spectacular battery life, so it's possible that the slight decrease that the Z4 suffers won't make much of a difference.
The processor has been upgraded to a 2GHz 8-core 64-bit Snapdragon 810, but it's the camera that Sony is really focused on, with 20.7MP, image stabilization, and features like "cooking mode" that helps you take better shots of your food. I don't even know what to say about that, so I'll go with nothing.
Rounding out the specs, we have a 5.2-inch 1080p LCD screen, a 2930mAh battery, and 3GB RAM. It runs Android 5.0 Lollipop and will be out in Japan this summer in white, black, copper, and aqua green. No other release dates have been announced.