Why 2 is Shorts and 4 is Trousers (pants)
Okay so here me out. I was listening to Doug/Parkzer talking on stream about the centaur pants options and I have some logic to suggest. (note i am British so when i say trousers think "pants" for Americans)
I think Parkzer was partly correct in that you cannot really assign human concepts like "pants" 1-for-1 onto something like a centaur that is inherently not human.
With this in mind, I want to instead start by focusing on the purpose of trousers/shorts which i think includes 2 functions:
They provide an extra layer to cover genitalia - if you're not convinced, think of it this way - in the hot days of summer (and not at the beach), do you go outside wearing just underwear, no, you don't because it's inappropriate, therefore trousers/shorts provide a function here.
They provide warmth to limbs/other body areas around the privates - I'm not being too specific here, because I'm thinking about many possible areas we use trousers to help warm, including privates, thighs, calf and potentially (if you wear high waisted trousers) even the lower chest/stomach area.
I also take function of shorts to be that they follow the above description, but with more exposed for better thermal regulation.
I also take one assumption for the Centaur anatomy - that the privates including butt and appendage of the centaur is at the rear of the centaur, like a horse.
With this setup in mind here is my take:
4 is trousers - it covers a large area of limbs/body parts around a horse, includes the privates and areas reasonably nearby
2 is a style/cut of shorts - it leaves more of the limbs/body exposed for better thermal regulation, while still meeting purposes of shorts above.
As a small note though, I considered the idea of possible other similar options not shown on image for leaving more exposed, such as imagine 4 but with shorter legs to leave lower leg exposed for example, this is why I say 2 could be a "cut" of shorts - just like with human shorts (think short shorts for example) there are multiple options for how you could design a garment that is more exposed than 4, while fulfilling requirements - different short styles.
And as noted, it's kind of trying to use human words to define something not for humans anymore at which point different styles or cuts of garments that fulfill the definition above might have different names in the centaur world - for example, maybe 2 could be called "Chorts" (centaur shorts) and 4 could be called Quadripants - but this is now entering fiction/fantasy territory.
Thank you for reading my ADHD inspired thoughts on centaur pants!