Being "very overweight" is very simple math - too much input and not enough output - eating too much and not moving enough. "Genes" and "conditions" and doctors and tests are all excuses and money making opportunities for doctors and slim fast diet pill manufacturers etc... Get that kid outside in the sun playing and eating healthy and he won't be hungry all the time. He's hungry because his body is saying it needs certain nutrients and if it takes 100 bowls of choco cereal to satisfy the body's x nutrient, he'll stay hungry. And he's probably unchallenged and bored and using food to compensate.
I wouldn't focus on the small instant cures (natural or not) but instead focus on his overall lifestyle, and what he is eating instead of how much he is eating. Healthy food satisfies the body, junk makes the body perpetually starved for nutrients. Most people still think healthy is eating Ramen noodles and diet soda (two of the worst things a person could possibly eat btw).
Most of all, these two sentences do not support each other:
"My sister is pretty good at monitoring his food"
"He is very overweight and just seems to be growing at a crazy rate."
The problem isn't the 4 year old - it's the person who feeds him. You don't need to spend tons of money and time to solve this problem.