If a crockpot/slow cooker recipe says to cook something on low for 4-6 hours, is it fine to leave it on for more like 8-10?

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.

It really depends on what’s in it. Beans, for example, will cook to mush if left on too long, but a soup would be fine.

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 wouldn’t do it unless whatever you are cooking has a very high liquid content.

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.