I taught CCD to high school aged students and I believe you are doing a very good job.
If your 4 year old raises questions you can not answer, this is a beautiful challenge for you. Take this as an opportunity to grow in your knowledge of the Catechism of the Catholic Church and to demonstrate to the child your formation.
Also, children are aware that we can not always answer all of their questions. They do not fault us for this. We should encourage their inquiry and, often inspiring, questions.
If you do not know the answer to the child's question, my personal suggestion would be to explain that you do not know. If you want to take it a step further, walk your child through the steps you take, personally, to find RELIABLE answers to questions about God and the Church. Your teaching by example may provide the framework for learning about God, independent research and finding answers based on Church teaching. What a wonderful gift!
Be prepared, however, to know that a child at 4 can easily be misled by others. It is always important to help the child to discern the difference between God's wisdom and other things that are not God's wisdom. Also, help your child to understand what to do, how to pray and call upon God's help and the help of the saints if he/she takes the wrong path or follows wrong advise, with or without good intent. Finding God's wisdom is not always easy, indeed, even for adults. It's a journey and, I believe, our duty as good Catholic Christians.
When I was a child I read the story about King Solomon and asked for God's wisdom. At that time, with my childlike understanding, I made a decision to ask for God's wisdom, not material wealth and personal pleasures. The result, I find, is that more I understand the things of God, the more I realize I know nothing. This is a Godly paradox. I believe it is a good one, a step in our journey. Do I believe I have the wisdome of God? No, of course not. But the end result of seeking God's answers to our questions is to draw nearer to God in love. In my personal experience, God provides guidance to me, when I am open to it, in His own time and in His own way, not my own. Often, I don't understand His direction. It's not my duty to understand, it's my duty to obey and love God. My mission is to strive to love God more, by seeking His wisdom and drawing nearer to His love and spiritual direction through the Holy Spirit and Christ, always.
So, for me, the act of searching for answers to Godly things, understanding the Catechism and finding Godly wisdom has a deeper impact on my personal formation than knowing the answers or thinking I found the answers. In the end, I find I know nothing. As we mature and seek God's understanding and Spiritual guidance, many times we don't find the answers we want or we can't see them now or God has other plans for giving us His wisdom. Does this mean we stop searching? No, never!
Important Tip:Waiting for God to respond in His own way, asking for His Grace and patience to wait for Him, is a true discipline. Mother Teresa of Calcutta teaches that silence and prayer is key factor in our spiritual relationship with God and growth.
Respectfully, I do not claim to have the "right" response to your question or the "correct" answer to your journey and the journey of your 4-year-old. However, may I suggest you lovingly show your 4-year-old the value of prayer and silence when seeking God and answers to any questions of God and our beautiful Catholic Church?
In conclusion:Understanding the Catachism and things of God is a journey, not something we accomplish. Even when we find the answer, that answer often grows and matures over time. Remember, we have an eternity to grow in the wisdom and love of our God, through our Lord Jesus Christ, according to His measure, not ours.
*God Bless you and I shall keep you in my thoughts and prayers. Ask Mary, if you will, to guide you toward Christ, she will, I assure you! *