Re: 3\" or 4\" PVC for septic
Highbeam has it correct, The larger the diameter the pipe the less slope it requires for draining. 4" diameter is the smallest used in the industry outside. The smaller pipes inside the house tend to grow in diameter as more pipes interconnect to prevent blockages and better flow. It is common in many houses to have 3" diameter main stacks, as well as 4" diameter. Outside should be 4" diameter, and if it will have a load on it like a driveway or driven on etc. use sch. 40 pvc. Pack the trench bottom with a layer of sand, it will make holding the slope easier while installing. Install a clean-out to the surface if the length is greater than 75 feet, in my opinion.
ps, a higher velocity within the pipe is not actually good, it can accelerate the "sanding" of the bottom of the inside of the pipe. A good flow is between 2 ft/s and 12 ft/sec. But that gets into hydraulics and I don't think this is that complicated of a project. Good luck with it. Have fun.
Dave