In fact, it’s hard to see where a Level 4 language perfectly fits. After you’ve surpassed the abilities of JSON or YAML (and you don’t opt for slapping on a templating engine like Helm does), it feels like jumping straight to Level 5 is worth the effort for the tooling and larger community.
https://developer.hashicorp.com/terraform/language/syntax/js...
Later on I migrated the solution to Terramate which made it a lot more maintainable because you write HCL to template Terraform config instead of JQ filters.
Although I really wish there was a first party solution or a well established library for this but I suspect that while it is easy to build only enough to support specific usecases but building a generic enough solution for everyone would be quite an undertaking.
HCL is best used when the problem you're solving is nearly one you could use a level 3 language for, whereas in my experience, Starlark is only really worth it when what you need is nearly Python.
Level 4 is also far more declarative by nature, you cannot fully compute stuff so a lot is abstracted away declaratively. This also leads to simpler code since you're less encouraged to get into the weeds of instantiation and rather just declare what you'd like.
Overall it's about forcing simplicity by not allowing the scope of possibilities to explode. Certainly there are cases where you can't represent problems cleanly, but I think that tradeoff is worth it because of lowered complexity.
Another benefit of level 4 is that it's easier for your code can stay the same while changing the underlying system you're configuring. Since there's a driver layer between the level 4 configuration and the system which can (ideally) be swapped out.