I have done 4 briskets. I've done Four 25lb Tom's. And I've done 8 10lb butts.
I've have found 4" of air gap is necessary / mandatory.
Put your 16lb packer on the bottom to the left, the second one goes in on separate shelf to the right, zig zag pattern to allow max smoke flow. The lowest brisket gets moved up to the top after (for me) 2 hours with the other three moving down.Do this when you spritz. I don't use my individual temp channels, just use a hand held instant read. After the stall, just rotate upper most and bottom brisket if you want.
If you are using packers get as much of the deckle out of the center between point and flat. Also on the thin end I shortened up the whole packer an 1 or 2. Some have less than a 1/2" of meat. Save all four and grind you a burger .
With a smoker full of butts grease can be a issue. I don't always trust the rendered fat making it to the bucket. My waterpan catches most of the dripping in my PBV5, but my Masterbuilt had an angle plate and oil drained to the grease trap. It clogged one time and I had a great big grease fire.
With a smoker full of turkeys I set my kitchen oven to 375 so I could zap the skin. But these need rotated, the bottom rack always runs hot.
If you cook flats just get up on em! There is plenty if verticle room/height.
I hope this helps.