How do I find documentation for the latest version?

I need to learn so I’m reading the documentation but I can’t find documentation for version 4.20.3, all it says is 4.9?

There’s no such things as documentation for 4.20.3. Stuff that applies to previous versions also applies to the most recent version. There might be some discrepancies, naming convention changes, deprecated elements but that’s all.

List of changes and new features for 4.20 only is here:

https://forums.unrealengine.com/unreal-engine/announcements-and-releases/1502911-unreal-engine-4-20-released

It usually takes a while for the documentation team to catch up. So if you want to find some solid info on Niagara, or rich text boxes for example, you’re out of luck, sadly.