conditional validation in Model

Yes, it is possible to have conditional validation in the Model using the when method in CodeIgniter. You can modify your validation rule for depth as follows:

'depth' => 'required_with[auto_build]|when[auto_build,less_than[4]]'
This will ensure that the less_than[4] rule is only applied when the auto_build field is present. You can use the when method to apply any other rules conditionally as well.