Best size for interior waste line

Hello everyone, long time reader, first time poster. I am building a new home and have reached the point of starting my interior plumbing. The home is 2 bath, standard fixtures, tub w/shower head, wc and vanity. Nothing fancy. I plan to use Toto toilets.
Anyway, I have, code here, 4" under my basement slab to the septic. There is a separate bath in the basement tied into this 4". My question is will I have more trouble with 3" or 4" for my lines in the home? I've heard pros for both and lean towards 3", but would like some more input.
I am fine with dfu for either, just want to gain some input on which will perform better at getting the loads to the septic.
Thank you all in advance.

Many codes require 4" when the forth toilet is added.
Also outside the foundation, many places want to see 4"

You do get better drain line carry with 3".

Thank you very much Terry.
I do have 4" for all of my underground. This has already been installed and inspected. I'm just looking for opinions of if I would be better served by reducing my 4"underground to 3" for the interior lines. To clarify, this would not be a reduction in the direction of flow, but rather an increase.
The home is a single story and I will have approx a 14' horizontal run from the farthest bath to the vertical stack, and both main bathroom branches will tie into this horizontal run.

3" has a better flow velocity, so it is better at scouring the pipe. The distances have little relevance to the question.

hj said:

3" has a better flow velocity, so it is better at scouring the pipe. The distances have little relevance to the question.

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Thanks for the input HJ
Better velocity would, I assume, go hand in hand with better distance per flush?
The septic tank is approx 100' from the foundation wall and is supplied by 4" underground.

Your code may be different, but here, if 3" exits the building , (i.e., only 3 toilets in the house), we run it all the way.