My experience with Sony Xperia M4 AQUA so far...

-overheating issues(you can literally cook eggs on the back of this phone)
-low gaming performance(even twice cheaper phone like the E4G has better performance)
-low available usable storage(under 2GB)
-takes forever to charge(4 hours via USB)
-takes forever for the camera to focus
-no way to root it
-no way to unlock the bootloader
-overall feels very laggy, unresponsive and slow

+waterproof
+decent camera

If i have to make a conclusion i'd say its pretty disappointing and suggest people to avoid it.