You're right! The fourth type of child is the one who doesn't even know how to ask a question.
How would you depict each of the children?
וְשֶׁאֵינוֹ יוֹדֵעַ לִשְׁאוֹל – אַתְּ פְּתַח לוֹ, שֶׁנֶּאֱמַר, וְהִגַּדְתָּ לְבִנְךָ בַּיּוֹם הַהוּא לֵאמֹר, בַּעֲבוּר זֶה עָשָׂה ה' לִי בְּצֵאתִי מִמִּצְרָיִם.
And [regarding] the one who doesn't know to ask, you will open [the conversation] for him. As it is stated (Exodus 13:8), "And you will speak to your son on that day saying, for the sake of this, did the Lord do [this] for me in my going out of Egypt."
Clue #3
Listen to the song in the video below. The words to the song come from the haggadah. Find where the words are written in the haggadah.