Yes when cooking in a crock pot on high for 4 hours it's the same as cooking on low for 8 hours.
When you want to really slow cook something you set the slow cooker on the low setting and the food will slowly cook within 8 hours.
If you want to cook the food a bit faster then you set the slow cooker to the high setting and the food should be fully cooked within 4 hours.
The higher the temperature the faster the food will cook.
Even when cooking in a pot on the stove such as cooking stew or soups the low heat setting would take longer to cook the food.
When you turn up the heat the food cooks much faster.
Some foods are best cooked when slow simmering than they are when fast cooking.