Did you perhaps make a nandroid backup within recovery?
Twrp recovery in and of itself is not that large and the recovery itself is not on the internal sd card, but nandroid backups are.
I have one backup on my external sd card that is 10.4GB.
I'm thinking what you are seeing is a nandroid backup.

If you have an external sd card, you can specify TWRP to save the backup to that instead of the internal sd card.

Where are you seeing this TWRP 4.23GB file?

 

Are you getting a "low storage space" warning? When do you get that warning? When trying to download an app from the PlayStore?
Try clearing the PlayStore cache and also from download manager:

You may have to do the same thing in Google Services Framework

 

This low memory warning happened after the casual root method?

As far as a factory reset helping? Yes, but you will lose your apps/data/contacts if not backed up.

Try clearing the cache and dalvik cache in recovery first.


I'm not sure what else to suggest there my friend