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
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..)