After creating a Database you will be prompted to manage the custom form behind it. This is where you essentially shape your database, based on the fields you wish to gather information on.
You can enable Conditional logic against a form field which means that it will only appear once a specific response has been entered by the user in a previous field.
For example, your third field could be a checkbox field with a Yes or No answer. If 'Yes' is selected, the user moves onto the fourth field. If 'No' is selected, then a new field will appear underneath asking another question.
To enable such conditional logic, once you've created a form element/field, when managing a custom form, select the question you would like conditions on then select Conditions. Check the box named 'Only show this field if conditions are met' and proceed to fill in your conditions using the dropdowns.
You can Add multiple conditions to suit your needs.
Additionally you can make certain questions 'only mandatory with conditions'. To apply conditions like this, select the Mandatory tab while editing a question and select 'only mandatory with conditions', then proceed to select your conditions using the drop down.
Remember to Save & Close to confirm your conditions.
You can also now add field based conditions that will prompt a whole Section to appear. Read more about that here.