Four sheets is enough, but six sheets would be better, especially if total solution is kept to 500ml or slightly under.
I like the simplicity and for travelling with minimal equipment space requirements and being able to develop 4x5 sheet film on the run, this could be ideal.
Rather than push for six sheets and dodgy evenness of developing, I would prefer four sheets and very even developing. If going to six sheets is pushing your agitation system, or enlarges the minimum tank volume over 500ml, stay with four sheets.
If the film holders were placed in landscape format, instead of portrait format, that could eliminate film movement issues caused by gravity or floating to the surface because of their buoyancy. This would require your unit to be turned 90º, just a thought. There could be issues with doing that, like scratching of the film by the guides.
The turbulator, or plunger rod, looks pretty much the same size as the Patterson chemical stirrer I have had for decades. My Patterson chemical stirrer has a 30mm diameter crushing base, but most importantly, the handle is designed to work well with wet or wet gloved hands. That handle design is so good, it has meant I have never looked for anything else to mix and crush chemicals.
http://www.bhphotovideo.com/c/product/40303-REG/Paterson_PTP245_Chemical_Stirrer.html
As someone who has mixed almost countless film and paper solutions from raw chemistry over the last three decades with a hand chemistry stirrer with a well designed handle, the holdability of your turbulator, looks like it could be bit iffy with a wet or damp rod.
With regard to solution volume size, I have been using my Jobo CPE2 for around 30 years, with 370ml of solution for six sheets (four works better) I always round up to 400ml and have had nary a problem with E6, C41, and all manner of B&W negative processes.
Having a little experience with injection moulding, I agree that the film holders are going to be tricky to manufacture, thin is difficult.
One last question, your link suggests that certain countries will not be shipped to, is Australia a ship to country?
Mick.