Where do you find the 4th red jewel in innocent life?

In "Innocent Life: A Futuristic Harvest Moon," the fourth red jewel can be found in the underwater area. To access it, you need to use the submarine that you acquire during the game. Once underwater, explore the area thoroughly, and you'll come across the jewel hidden amongst the coral and sea life. Make sure to check all corners to ensure you don't miss it!