PnP Modern Search v4 is still under development and there are no events when the search is changed or the results are updated. 

Until these events are added, if you use the 'customTemplateForPnPv4.html' template, SmartPreviews will display the binoculars button for all results, even if they don't have a supported extension (for example: aspx, html and so on) or if according to SmartPreviews rules a preview wouldn't be created for the document. When clicking on the binoculars of such a result, the document viewer will display a generic error "Oops! Something went wrong. Try again later.". For the documents that comply with the rules, the preview will be generated and displayed. 