This question may be obvious, but after a while trying it, I wasn’t able to do so.
How do we change a 5-line staff into a 4-line staff?
The short answer is that you can’t. It may be possible to bring one in via MusicXML hackery (search; this has come up before) but there’s no guarantee that Dorico will handle a 4-line staff the way you’d like it to, and there’s no dialog in Dorico in which to take control of these decisions.
I see. I think this is a feature that should be available in the next version of Dorico. I use it a lot.
You can hack your instruments.xml file to do this. In Windows it’s found in Program Files/Steinberg/Dorico (whatever version you are using). Be sure to make a backup version of it first. Take whatever instrument you want to have a 4-line staff and change the entry from 5 to 4.