Cast is 4”…replace with 3”?

[Thorngrove]

I’m replacing all the cast iron waste pipe…2 sinks, 2 toilets, 1 shower. All fixtures connect to 4” horizontal which runs to main stack.
Question…can I replace with 3”? The stack will remain 4” though.
Also…a toilet is directly above the current horizontal run…it currently uses a sanitary tee on its back, the way I understand things, shouldn’t it be a Wye instead? I’ll attach some pictures cause Im sure I haven’t explained things well enough. First time being on a forum and asking a question…so I do appreciate any guidance and thank you in advance.

 

[Tuttles Revenge]

Yes, 3" pipe is rated for 35 Drainage Fixture Units or DFU. Toilets are 3DFU if they're 1.6GPF or under. I believe that KS uses the newest UPC code which allows up to 5 toilets on a 3" horizontal.

And yes. Long sweep fittings for all transitions from Horizontal to Horizontal and Vertical to Horizontal. The toilet shouldn't drop into a horizontal connection either. Post a drawing of your layout of the fixtures / walls / point of connection in relation to the fixtures.

 

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[Sylvan]

A few decades ago a toilet would discharge 3.5 -5 GPF and ld shower heads and faucets had no flow restrictors so the piping was sized by fixture units (7.48 gallons)

Now with low flow fixtures 3" would be fine as you will have better scouring conditions