You can "just" fit A4 paper in there if you move the divider so it attaches to the side walls and not the back seam. That said, a folder that added even 1cm would make the bag too small. I wouldn't recommend this bag if you needed to carry A4 paper.
My favourite camera bag is the ONA Prince St messenger bag in dark truffle leather - it's luxurious and a real pleasure to carry around. That said it is for my carry everywhere X100T, and it wouldn't hold mush more than a very basic system (eg. one body, 2-3 lenses).
My preferred bag for work is the Thinktank Airport Airstream. It's international carry on sized (meaning I can use it in Australia), and it carries a lot of stuff. Here you see it setup to carry my gear, and here's what's inside (clockwise from the top):
4 x X-T1 (2 with battery grips)
Nikon 35mm f/2 under a 50mm f/1.8 on a Kipon tilt adapter.
10-24mm f/4
50-140mm f/2.8
18-55mm f/2.8-4
56mm f/1.2
23mm f/1.4
35mm f/1.4 on top of stacked 11mm and 16mm extension rings
16-55mm f/2.8
Nissin i40 flash unit with spare batteries and diffuser
Rokinon 12mm f/2
Vello off camera flash cable
Then in the lid I've numerous small accessories/tools etc as well as 8 x W126 batteries and a card pouch.
One day I need to lay it all out next to the body so it can be truly appreciated!
