These images are basically a comparison between the metal frames of the Z3 and the Z4 which shows that they both have the same Sony Omnibalance design though he Z4 is said to be 1 mm thinner.
More images after the break.
Aside from that, there doesn’t seem to be many differences between the two smartphone chassis, but we have expectations that the real changes will be under the hood. Perhaps with a better display and faster processor than the Xperia Z3 at very minimum. We’ll find out soon enough hopefully.