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.