im M4 user.. u can feel warm on camera lens area during heavy usage or some apps like Chrome, GPS (by mobile data).. last week i did play a lot of AR effect apps with my nieces.. same feel warm/heat but not until not comfort to touch.. because warm area only located on camera spot there, i believe motherboard and CPU located there as well.. the body of phone is good to hold so wont affect normal usage..
8gb internal storage is the big concern for everyone including me.. but i know the purpose and how i use a smartphone for now.. so i think its still sufficient to me currently, but not in future (unless got root)..
battery life (wifi on, use FB, chrome, youtube, whatsapp, line, messenger can go up to 6 hours Screen on time for 1 day frm morning till night) (mobile data can be 3.5~4 hours screen on time depend on coverage and signal)
battery life normally nt a much issue for M4.. me charge my phone every 2 days currently with 2 hours screen on time per day with wifi on at home and office (smart connect setting for everyday auto wifi on off).. so 2 days usage, screen on time 4 hours.. i would say battery life is great for M4..
camera is good.. even in lowlight, the output is good.. not that blur.. focus area always good one, out of focus area is blur.. can check some of samples in previous page in this thread.
performance, i dont know how its perform in gaming part since i just use for daily usage.. Uber, FB, LINE, whatsapp, chrome, youtube, etc... no problem or lag found (except enter to setting>storage>a bit laggy feel in storage or apps page.. due to system trying to compute the size of apps) but i think can be further improve in future, cause no lag found in my ZL..
GPS is good.. cause everyday im using it.. Uber app, google, papago.. but waze cannot use.. Even waze also announced that they are finding solution for Xperia M4 aqua.. up to the latest apps, there are no map showing in waze yet.. so this might be some concern for users.. temporary im using google map to check traffic info..
web browsing (with wifi connected, there are minimal warm you can feel, but with data on, my home celcom connection not so stable, so will feel the warm after a couple minutes)
Didnt expect for a so detailed advice.
Wow~ the upcoming C5 & M5 are soo tempting..espcialy the C5 will be the great threat for the newly release A8
BTW, what abt the xperia T line up..dropped already? IMO, it's good that Sony keeps their product line up tightening.