I think you can have a pretty good guess at what the Z4 will be. Similar design with some minor improvements, similar hardware to the M9, Sony sensor.
IMO unless IP certification is a must have feature for you, there likely isn't any other area it'll be better than the S6.
- I don't think the Z4 will have biometrics unless they significantly change the design. The glass back also seems to make a back placement unlikely. With the S6 gaining touch biometrics like the iPhones, there's now a choice in both ecosystems for this feature.
- The camera will likely follow Sony's past efforts and be good, but not as good as the Samsung equivalent using the same sensor. I feel pretty good in guessing the S6 camera will have better photos, especially in low light, and be faster to launch and focus. However, the Z4 will certainly be competent. It's almost infuriating considering Sony makes the sensor + has amazing camera expertise that the cameras in the Z2 and Z3 have underperformed compared to competitors.
- IP certification will remain a strong plus
- Display will be good - no idea if it'll be 1080p or 1440p. Considering how poorly the S810 handles even a 1080p handset like the M9, I'd be concerned if Sony used the S810 + 1440p screen.
- Battery life will regress. No fault of Sony, but the S810 just isn't very efficient. HTC has been good regarding battery life with the M7 and M8, but even with a 10% larger battery and similar screen, battery life has regressed substantially.
- Will carriers sell the Z4 in the US? So far it's been pretty spotty and Sony's recent drama may not help.