Use of a Sanitary tee vs wye combo

Can someone explain to me why a sanitary tee on it's side on a horizontal branch is not acceptable. From what I am reading a tee wye combo is what is supposed to be used. My old house has 4" C.I. and the toilets have a short lead bend directly under the flange and then they are connected to the horizontal branch with sanitary tees. I am going to replace the entire branch with PVC. If I drop down to 3" I probably can use tee-wyes...If I stay at 4" there is no way I hve enough room to make the turns. 40 years it has worked and never had an issue....of any kind. I would prefer to stay with the 4" as when I look at the 3" PVC...it just looks like a clog is more likely with the smaller I.D. ????

bend

It is a matter of hydraulic flow dynamics. It was not proper when it was installed, but must have slipped by somehow. Depending on the space involved, usually all that is required to do it correctly is to rearrange the piping slightly.

See the Charlotte Pipe catalog, ..http://www.charlottepipe.com/Documents/DimensionalCatalogs/Plastic_Pipe_Fittings.pdf.. look at San Tee on p. 23, Combo on p. 30, and wye on p. 33.