To root m4Aqua E2303 you need BL unlocked, but before, it is best to do backup.
Then after flashing boot.img, recovery.img and SuperSu, M4Aqua is rooted.
Finally it will be usually systemless root check with root checker. It works fine on Android 6.0.1.
Since I had some troubles rooting my Xperia M4 Aqua the other day, I made a video tutorial on how to root it, because it was kinda hard for a newbie like me to root it with the help of other posts on the forums. The tutorial is a step by step tutorial, and I hope it will help newbies like me to root their phone.