Just to clarify, that setting is for tabs, not for YAML lists. And it’s just a visual thing. Pressing Tab doesn’t actually insert that number of spaces; it inserts a tab character. The setting just determines the width that you see on your screen when you insert a tab character.
The difference is important to know because YAML specs explicitly do not allow tab characters for list indentation: YAML §6.1. Indentation Spaces. Also, there’s no proscribed number of spaces.
A solution (or workaround) for you depends on what you want the spaces for.
You can create YAML lists with four space (or one or any number that’s consistent within that list) and the Properties interface will understand them as a list. For example, this is okay:
---
thisIsAList:
- a
- b
thisToo:
- c
- d
---
But once you use the Properties interface to add or edit any property, Obsidian (for its own reasons) changes that note’s YAML lists to two spaces:
---
thisIsAList:
- a
- b
thisToo:
- c
- d
new: I added this unrelated property using the Properties interface, and all I got was standardized list indentation.
---
So if you need the extra spaces for entering or importing the info (which is one of my frequent use cases), then you needn’t do anything differently. Having more spaces will work fine.
However, if you want to create or preserve four spaces for exporting or copy-pasting to note content, then you might be stuck with workarounds like doing a replace-all (after exporting or after pasting) or not using the Properties interface (instead using Source mode or an external editor to edit the front matter).