While we wait impatiently for an announcement at the Mobile World Congress, where it is expected that Sony will give the Xperia Z4 a formal introduction, it looks like the device is starting to make the rounds with the relevant regulatory authorities.
The Japanese Communication Ministry has apparently given its stamp of approval for the Sony Xperia Z4, and will be available with NTT DoCoMo, KDDI, and SoftBank. All three carrier versions support Voice-over-LTE, and other emerging features that are making their way to the mainstream.
As for what we will see in the final Xperia Z4, the rumors have been hazy, and the news from Japan is not clearing up any of those rumors. It is possible that the Z4 will continue with the current current form factor of the Z3, meaning a 5.2-inch display but with more compact bezels. That would give plenty of room for the rumored Z4 Compact and Z4 Ultra, but again, this is all conjecture.
However, if the Japanese version of the FCC is already looking at a device to give the thumbs-up to, we expect to see news of similar filings being made in Washington, DC for a US variant (or three) of the Xperia Z4.