My needs are very simple - can I start using CI4?

Yes it does make CI updates a lot easier.

For unrelated sites or apps, I'd still probably use individual CI installs, as I wouldn't want to randomly break sites. Better safe than sorry, I guess.

In our case it was multiple "mini-sites" that needed to be on it's own just to make code and front ends organised better, but it also had few shared models and what not that I didn't want to keep having to update between different projects/repos.