Sony Xperia M4: is the Sony bloatware installed on the system partition or the data partition? As detailed in my previous question (How to upgrade a phone with only 8Gb. internal memory), a relative of mine has a Sony Xperia M4 with Android 5.0 and only 8 Gb. of internal memory; after installing the few apps he uses, he has now only 320 Mb. of internal memory left.

Of course, the phone comes by default with all the Google bloatware AND all the Sony bloatware, and they can't be uninstalled by default. I'm considering rooting the phone to delete the unwanted apps, but this lead me to think: and what if all of the Sony bloatware is installed in /system, and deleting them doesn't free space in /data, which is where the space is needed?

So... does anyone know where are the Sony apps installed by default?
"can't be uninstalled by default" is a very obvious indicator that they're in /system.

You are correct about that deleting them won't free space for /data - unless you somehow find a way to repartition the storage and allocate more space for it, which is beyond normal users.