Toilet arm pipe size

I am doing a complete replacement of my existing 4" cast iron drain system in my old house to correct some existing problems and later add a bathroom and kitchen in the basement. I have done extensive research through plumbing books and internet forums and am confident that I can so this correctly, but I read something in one of the books that raised a question. Should I use a
3" wc arm for the new low-volume Toto toilets I plan to install instead of the 4" size I was planning to use so that sufficient velocity is maintained to insure that the solids move with the liquids? The existing 4" wc arm is close to the vertical stack and the proposed basement wc would drain into a new horizontal branch. The plumbing books encourage use of pipe sizes larger than minimum when possible. I know 3" is minimun for a wc arm, but I assumed 4" would be better.

If you have the room, stick with 4"...as long as the slope is proper, you won't have problems with either.

waste

With drainage, bigger is not always better. In your case either 3" or 4" is adequate, but 3" will give better performance in the long run.

Thanks for the answers, but I am still confused. I do have plenty of room for 4" under the existing main floor bathroom - why would 3" give better performance in the long run? I have seen that recommendation before but I but feel reassured that it is correct when it is coming from a Master Plumber. I have read in some forums that 4" is required for wc arms under a concrete slab by some local codes so I will have to address the question again when I get ready to do the basement. For now, I just want to make sure I do the main bathroom the correct and best way because I am only going to do it once and don't want any problems down the road. I didn't mention this in the original post, but the existing problems I am going to correct are an
11" rough-in and a toilet arm that slopes down from the stack toward the wc, rather than down toward the stack at the correct 1/4" per foot.

pipe

Flow velocity creates the self scouring action of a drain line. If the line is oversized the flow slows down and then materials can accumulate in the pipe. Some of the worst stoppages are when you have an oversized drain line with grease flowing through it.

Thanks again, hj. I think I'll go with the 3" toilet arm for the existing bathroom to insure good scouring with the discharge from the new low-volume toilet.