unityscript - The value of VSync on vSync Count in Unity

In Unity, we can set the vSync value as "No sync" = 0, "Every V Blank" (60 fps) = 1 and "Every Second V Blank" (30 fps) = 2. But in the Unity Quality Setting documentation , it's written the vSync count value must be either 0, 1, 2, 3, or 4. What is 3 and 4 ?

You can set this vSync value on your code, the value must be either 0, 1, 2, 3, 4. But in Quality Setting in Unity, the option is only for : No Sync, Every V Blank, and Every Second V Blank. So basically only for 0, 1, and 2.

There's no option for 3,4. If we set the value on the code as 3 or 4, how should we set the value in Quality Setting Properties ? Because we need to choose a value other than No Sync for the vSync to work. How the vSync count in Properties and Code related if they accept different value ?

Answer

It's written in the link you provided:

The number of VSyncs that should pass between each frame.

So, 3 means one frame every 3 VSyncs (with VSync at 60Hz, this means 20FPS), and 4 means 15FPS.

EDIT: to answer to your edit - the value you set via the Quality Settings is your "default" setting, if you change that value by using QualitySettings.vSyncCount anywhere in the code, the latter will always takes precedence.