You can include BS 4 from a CDN as explained on https://www.w3schools.com...trap_get_started.asp

You could include this using the site settings (page header) whcih would enable BS 4 on all pages. It might be too much to load 2 versions of Bootstrap every page so, you can also use the page settings (advanced, SEO, page header settings) or even the header in the module settings. All depending on how many times you need it.

There is always a risk of conflicts between bootstrap versions.
In principle mixing versions is not supported by bootstrap (and related to DNN).

What someDNN vendors have done is create a custom BS4 stylesheet with all CSS "wrapped" in a root class and load that

<code>.bs4 .......</code>

That does not help against potential JS confict though, but in some cases you can also remove bootstrap js from the skin.

If you want to avoid conflicts your best option is to "upgrade" the skin to BS5.
I have upgraded some skins before and for most skins it's actually not that difficult.

(unless it's one of the commercial 1000000 options skins..)