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"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
a 100mm dia pipe will do, just make sure the slope is maintained with in the range of 5% to 13% and fittings (45 & 90 deg) bends are installed in consistent with the plumbing code.
Above answer, is best preferable consult local building regulation.
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" To infinity and beyond" - Buzz Lightyear
It appears to me that this member is trying to size a discharge pipe for a facility having 4 toilets. Usually a 4 inch discharge line is used in houses with more than 3 toilets. The grade is 2 percent to the septic tank and the laterals are usually laid at 1% slope. The grade is kept so small to prevent the liquid from outrunning the solids..
Good answer except the questioner is running to a manhole which presumes a community sewer system. In which case the lateral is the line from the lot to the main and is usually kept at 2% also, Then the sewer mainlines will run at 1% due to their increased size and thus need to maintain flow depth for consolidation of fluids and solids. Just make sure it runs downhill and don't chew your fingernails.
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