Gary take a look inside your current electric portables and see how much extra stuff is in there apart from just the vacs and pump. There's capacitors and other malarkey that I don't really understand fully, but you have to understand it's all there for a reason so I wouldn't just wire up 4 vac motors to the mains and hope it works! What I would do is acquire two twin-vac machines that are being scrapped and take the wiring looms out of them.
Don't forget you'll need to consider cooling - twice as many vac motors = two cooling fans and plenty of thought to air flow in/out
In the past I used to run 2 ninjas paralelled together so have run pretty much the same configuration you're planning... it worked really well and that was only with 1.5" hose! Don't choke your system with small bore plumbing - give your "box" two 2" vac outlets and fix a second 2" port to your waste tank.
There is one BIGGGGGG problem here though. Those tanks you've bought are designed to be used with a portable that has its own waste tank and its own means of keeping foam and debris out of the vac stacks. If you just build 4 vac motors into a box and connect them on to one of these tanks, you need to put some serious consideration into this problem. You don't want to just connect a pair of vacs straight through to the tank because you'll be blowing vacs left right and centre.
Just an idea that's popped in to my head... you could butcher a Ninja or something similar..... cut the vertical back panel off the fresh tank and mount your extra pair of vacs in there, soehow use the ninja's waste tank with stack pipes and ball & cage shut-offs. Hmmm, thinking as I type, might not be very easy.
You could adapt the scorpion tanks with two stack pipes that come up through the bottom of it, basically the same as in a ninja or similar. That should work
I bet this is going to be a lot more complicated than you first thought
