I am trying to translate the English expression "left to their own devices"/"left to themselves"/"in a vacuum" into Japanese, and the best I could come up with would be something involving 「ひとりで」, but I am trying to describe a group of people who I am treating as a unit (effectively 一族). Is there any way to say "alone" when talking about a group of people (in this case 四人), as in "without the influences/presence of an out-group"?
Where ひとりで/1人で works, 4人で also should work. 4人だけで might be better if you want to indicate 4人 is a small number. A more generic and neutral expression would be 自分たちだけで, but I'm not sure if this is usable for the sentence you have in mind. 孤立して or 隔絶して might be even better in certain contexts (e.g., living in an desert island).
After poking around I found this webpage with a translation of 'left to his own devices'.
To quote the site:
There are possibly other translations that could be used in context, but that seems to be adequate as a general translation.
Other specific examples are:
If left to himself, he will drink himself to death. (Weblio)
Children are left to their own devices to play. (Linguee)
Leave me alone! (Weblio)
Left to themselves (i.e. no supervision). (Weblio)
The best translation will depend on what fits with the whole sentence.