I have four children but only 3 of them showed up in the earned income credit section, why is that ?

You don't need 4.   3 or more is the highest amount on the table.

https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-credits-deductions/qualifications-earned-income-credit-eic-eitc/L7w4BFP32_US_en_US?uid=ldcbqmwd  

You cannot get EIC for more than three children no matter how many children you actually have.  That is the limit.   The child tax credit is different--- you get that credit "per child" for as many children that you have who are under the age of 17.   The amounts of each of the refundable credits is affected by how much you earned by working.