YITH Wishlist 4.0 includes new React components to improve the plugin performance, so if you would like to take advantage of them you will have to convert them:
When clicking in the related message, you will get a popup which will give you the possibility to either preview the React templates in your site, convert them directly, or not proceed until next update.
If you click on preview, a new tab will open where you can test how the components work on your site:
After testing, you can decide whether to proceed and convert the PHP Templates to React or close the preview and continue using the PHP version.