Not if the directions specify low for 4–6 hours. If it said high 4–6 hours then yes you could cook it on low 8–10. If they say low and you cook it double the time, expect a very dry, possibly burnt meal.
There are some foods that can be cooked longer, but generally the temperature should be at the lowest for longer. With the higher temperatures it could be overdone.
I challenge ”...a soup would be fine” with personal experience. I’ve had the flavor cooked out of soup by leaving it on the stove for too long. I wouldn’t recommend leaving anything cooking for that much longer than the recipe says.