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 logic against a form field whereby it will only show when a certain answer has been previously selected by your end user.
For example, your third field is perhaps a check box asking a question 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'd 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 drop downs.
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 Mandatory while editing a question and select 'only mandatory with conditions', then proceed to select your conditions using the drop down.
Remember to Save or Save and Continue to confirm your conditions.