About Bootstrap Version Usage

The extension detects if you are using the yiisoft/yii2-bootstrap or yiisoft/yii2-bootstrap4 and loads the appropriate assets for the extension. It will check first the "yiisoft/yii2-bootstrap4" if it exists then it will load bootstrap4 resources otherwise it will fall back to use bootstrap3. So make sure you are following the correct guide to use the yiisoft/yii2-bootstrap4" and remove the "yiisoft/yii2-bootstrap": "~2.0.0", from you composer.json and change the minimum-stability:"dev" here is the complete guide.

You can still force bootstrap version for the widget if you are using both of them by using the option forceBsVersion in the main options of the widget see docs