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Your question is long, but it sounds like you are simply asking if the tub drain will fit within the 4 inches you have to work with; please correct me if I am wrong. If this is the case, I believe that you can fit it; it is usually the toilet that is the hardest to fit due to the larger piping required. The diagram below, showing a cutaway view of a tub and drain will give you an idea of how the tub should be drained. Since there is a trap involved, the vertical distance from the tub drain to the end of the trap can be within the 4 inches, easily. The diagram shows the trap low, but you should install it higher.
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Best of Luck, Brian
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All drains should be vertical or pitched at 1/4" per foot.
I will respond back in a few minutes. I am going to scan a picture of a similar bathroom setup showing all of the plumbing, so that you can better understand how to connect everything. It is easier than trying to describe it with words, which would take several paragraphs.
This is an article that I was just browsing in Fine Home Building. It shows a basic layout and how the drainage components connect to each other. Ignore the sewage tank and pump in this picture; the drain line would keep going to your septic connection, in your case. Your fixtures can be located in different locations, but you can still use this picture and apply it to your layout. The vents are shown, as well, and these are typically inside of the bathroom walls. The vent pipes can become one and will need to vent thru the roof; getting to the roof can be done by travelling thru closets, etc. on the floors above.
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