How to increase user storage space in Mi4i?

Recently I was flashing PAC rom. To do that In recovery (TWRP) I wiped every thing. All the options in Advance Wipe in recovey except USB OTG. When I checked the available space, it was showing 12 GB available. Without flashing the ROM. :(
Mi4i is a device with 16 gb storage. So, there must be reserved space for ROM to install.
AOSP rom will never use 4 GB of space, So from that 4 GB reserved space, there might be 2 GB of space still available.

Is it possible to resize the partition table and increase the usable space?? :confused:

Any help will be useful?
Thank you in advance.......