Can I cook a slow cooker recipe on high instead of low, but for shorter time?
It will be safe and edible. It might not be quite as good.

Part of the appeal of slow cookers is just the convenience of leaving them unattended. The other appeal is low-slow cooking that blends flavors and melts connective tissue without burning anything.

Meat
Cooking things faster and hotter will not make the meat as tender as it would be- but it will still be cooked. If it is in small pieces the difference will not be as noticeable.
Obviously ground meat would be the same regardless of cooking time.

Beans (if you believe in beans in chili)
4 hours will be enough time to soften soaked beans. I'd say this is the biggest risk. Undercooked beans aren't fun to eat.
Soaked beans, canned beans, or no beans make this a non-issue.

Flavor melding
Chili benefits from time. Beans soak up flavors. Chili and onion balance out through the sauce.
I'll often make my chili a day in advance of serving it because it is so much better the next day. You're going to get less time so some of this will be lost.