I have known for 30 years half the secret to having the same feelings and wonder as when I was 4 years old: Gift of the Holy Ghost (as maintained by real prayer and study and application).
Flashbacks induced by the sense of smell and given by the Holy Ghost help.
Today I realized the other half: Eat things such as raw buttercup squash so your body can get into it too, not just your mind, and have the same boundless energy you had back then.
I FEEL ALIVE! As alive as that fresh buttercup I killed this morning. Before I killed it.