Hello malangon,
First, I don't know what your location is like, but one problem you can encounter with using the 4 inch tubing is heat buildup in the root zone. Especially if the tubing is in direct sunlight. Ran as a flood and drain system cool water floods the tubing and root zone, but in the off cycle the cool water drains back to the reservoir, and sitting in the sun heat builds up in the root zone. Then it cycles cool water through the tube again during the on cycle. So you wind up with cycles of cool and hot in the root zone. Now constant flowing water can help alleviate the heat buildup in the root zone, but the just like when cycling it, the reservoir temps will slowly rise as well. So if you think there may be a problem with heat build up, you may want to think about insulating the tubing/root zone. I don't know if you know but in case not
nutrient Solution Temperature is Important" target="_blank">Nutrient Solution and Root Zone Temperature is Important, that was my first major lesson when I started growing hydroponically.
Second, I wouldn't grow all those plants in the same system. The reason is because you have many widely varying plants. Some flowering and some not, some large and some small in comparison, some long term and some short term plants.
Third, I wouldn't try and grow tomato's or peppers in a 4 inch tube. Tomato's and peppers are large plants that will have a large root mass. While plants will always try their best to adapt to their environment, the large plants will quickly outgrow such a small root space, and eventually clog the water flow in the tubes. I prefer to grow large plants in a
drip system because I can easily make the root space any size I need, and you don't need to flood the entire root space to wet the roots. I prefer to use coco chips as a growing media for three reasons. One, because it holds moisture well withough't becoming saturated, and because the larger partial size of the chips leaves larger air gaps between the particles. Giving better aeration to the roots for the best of both worlds (water and air). Lastly because it's cheap
Fourth, lettuce is a short term plant, so depending on how much of it you plan to grow, I would probably grow it in it's own system. It would be a good candidate for the 4 inch PVC tube system you plan on. Although I would use ADS tubing instead. ADS tubing is black on the inside to block light, and white on the outside to reflect light. Most lettuce do well in water culture systems, so running it either way (as a flood & drain system, or recirculating water culture system) should work fine. If running it as a recirculating water culture system I would keep the water level below the baskets to keep an air pocket between the plants and water level (similar to a NFT system but higher water level), simply because it's hard to get air stones/blubbers in the tubing to aerate it. If running it as a flood and drain system, I would try an keep the on/off cycles short to maximize air intake, and maximize water circulation/movement.
If running it as a recirculating water culture system, There isn't to much need for an air
pump in the reservoir because the water is circulating 24/7 and the falling water helps aerate the water itself. Also if you leave an air gap between the plant baskets and water level, the plants have a secondary source of air/oxygen. If running it as flood and drain system, I would use an air pump in the reservoir not only to help aerate the water, but more importantly to circulate the water during the off cycle and help reduce the likelihood of pathogens gaining a foothold in the system.
Fifth, as for the Spinach and Basil, these are both non flowering medium size plants. Well they do flower, but you don't want them to flower because your growing them to consume the foliage. Again while you can easily design any type of system to grow these well, I think my preference would likely be in a drip system. Mainly because their long term plants, and simplicity. Also I think the spinach likes to spread out like ground cover, and I can easily provide more surface area for it to do that with a drip system. However depending on the space I had to put them, I might grow them in a low pressure
aeroponic system. Similar to what you described as your raintowers.