Does any body know if large plants up to 4` will be supported ok in a 6" hydro/aero pipe system, or would i have to go one better and use 9" or 12" Pipe,
For plants that tall (4 feet) in any tube type system you will probably need to build a trellis to support the plant. The tube should be large enough to hold all the growing roots without clogging, but the plants can easily be supported by being tied to a trellis if needed. Depending on how many plants, no-matter how big the the tube, it can easily topple the system over simply by the weight of the plants.
also what size tank, how long pipes, and how far to space the plants apart 40 plants total.
This depends on the size of the full grown plants. If the plants are 3 feet wide when full grown, I would space them a minimum of 3 feet apart (probably 4 feet). They still need elbow room, but the spacing depends mostly on the type of plant. The size tank depends on a lot of factors. Are you running it as a NFT or ebb & flow system (flood and drain). But for 40 plants you will need a large reservoir regardless.
A flood and drain system will need much more volume in the reservoir. First you would need to figure out how much water it will take to flood the system, this will depend a lot on how you build (or have built) it. Then I would at least triple that amount, so it wont take any more than one third the reservoir to flood (I would use this as a minimum). A NFT system does not need nearly the same amount of water to flood the system.
But keep in mind that each 4 foot tall plant (depending on weather)will drink up more water the bigger it gets. At 4 feet tall each plant will probably drink up between 1 and 2 gallons of water daily. 40 plants x2 gallons each, that's about 80 gallons of water you will need to replace daily. As the water is used up the nutrient
solution becomes concentrated (unbalanced), that will lead to nutrient problems. Not to mention constant pH adjusting. I would probably go with something like a 400-500 gallon reservoir, plus whatever it takes to flood the system. I might even split the system into 2 or more different reservoirs, in case I wanted to grow different crops in the same space.
I would create a water replenishment system, using a float valve that opens (like in a toilet tank) when the water gets low. This would need another reservoir that held the fresh replenishment water in it (pH adjusted of coarse). This wont keep the plants from drinking up the needed water, but help keep the nutrient concentrations from fluctuating so much. This replenishment tank can hold plain water, or a very diluted nutrient solution.