If you're just setting up Hubitat today, your only option is Rule 4.0, so no need to make a decision. 
(Actually, there are still lots of decisions to be made, mostly: is Rule Machine the right tool for the job? It's a bit complicated and requires some learning, especially if you're not familiar with how other aspects of the platform work already, so I'd strongly consider utilizing the vast array of other built-in apps instead initially if any meet your needs.)
If you're reading the docs (please accept my applause), then you might just be running into an issue of naming: technically, the parent app is "Rule Machine" and the child apps (rules) you make are "Rule 4.0." If you're in the docs wiki, the Rule 4.0 docs are what you want (though I think the community post is better with inline screenshots). The Rule Machine docs in the wiki contain general information that's more helpful if you're writing a custom app to interface with its API than it is if you're a regular user of RM. Informally, people use "Rule Machine" to refer to all generations of parent and child apps (like I just did), so don't get too hung up on this distinction otherwise.
Oh, and welcome aboard!