Well the Z3 series has only been out for a very short time but rumours about the follow up Z4 series has already started. Sony appears to be continuing with the "2x flagships a year" release cycle, with the Z4 series of products said to be possibly announced at CES in January 2015, but will probably be more likely to be at MWC in March 2015 as that fits more with the 6 month cycle (Z3 announced Sept 2013).
It is rumored the Z4 series will include a Z4, a Z4 compact, a Z4 ultra, and a Z4 Tablet.
They may all be powered by a 64bit Snapdragon 810 Octacore (big.LITTLE configuration), Adreno 430 graphics, 4GB RAM, 32GB onboard with microSD expansion, Qi wireless charging instead of the magnetic dock, improved camera with a focus on 4K that doesn't cause the device to overheat, improved lowlight performance, and removal of the USB port flap while still maintaining IP68 rating (similar to how the S5 mini has an open USB port but is still water proof). Also rumoured is a redesign with smaller bezels while keeping the dual front facing speakers.
Z4 Compact - 4.6" 1080p
Z4 - 5.5" QHD
Z4 Ultra - 6.4" QHD
Z4 Tablet - 10.1", rumor is it is being tested with a UHD 4K screen, but will likely only come with a QHD screen
If these turn out to be remotely true, there will be a couple of device here I will be getting. So anyone else really excited about any of these?
a few related links
http://www.techradar.com/news/phone-and-communications/mobile-phones/sony-xperia-z4-what-we-want-to-see-1268100/2#articleContent
http://www.trustedreviews.com/news/sony-xperia-z4-release-date-rumours-news-specs-and-price
http://www.gsmarena.com/rumors_on_xperia_z4_promise_sharper_screens_better_designs-news-10191.php
http://www.phonearena.com/news/Latest-rumors-about-the-Sony-Xperia-Z4-Xperia-Z4-Ultra-Xperia-Z4-Compact-and-Xperia-Z4-Tablet_id62753