Triangulating Meshes

Hi. Yes, always in ts4 and most likely in all games.
It takes at least 3 vertices to create a surface, and 3 create a triangle-shaped one. No matter where in space they are, they always lie on the same plane and it is shaded flat. When the surface is made of more than 3 vertices, its shading waries.
On the first image, the quad appears flat and is shaded flatly. When the angle is changed, you see that it is not actually flat and can be shaded in two ways. When you turn this quad into two triangles, there is no ambiguity and that what speeds up the rendering of the scene. The performance is very important for gaming engines, so  that is why all game assets are triangulates

This may not seem like a big deal on a simple mesh, but it is very noticeable on low-poly meshes of organic shapes like clothing, humans, basically anything that's not angular



But to answer your s4s specific question, s4s triangulates the mesh itself upon import, you do not need to do it yourself unless dealing with alpha hair or if s4s does it in the unfortunate way