This is the question that my 4 year old ask me one day after having our devotions. My 4 year old is very inquisitive and I really like it that he's made this way. He ask the most unexpected questions and he doesn't let go of a subject until has has fully understood it. And I tell you, it's quite a challenge explaining spiritual things, concepts or ideas in a way that a 4 year old can understand.
As with his question, Who made God? I was able to explain to him that God has no beginning and has no end. No one made him.. and He exists by himself.. and told him all the attributes of God. I was a bit worried that he might ask me to define 'exist' and all the other words that I said that he might not understand.. but thankfully, I saw him nodding his head and said, 'Okay!'.. and no further questions were asked. :)