Create Script for In Slideshow Gallery Adsense Advert

I have some image slideshows that get a lot of hits. I have seen in image advertising services that have an advert every 4 images. I currently use google Adsense and another provider of adverts. They are great on the content elements but unfortunately (or fortunately) our site has 80% mobile views, so the slide-shows get no revenue whatsoever. I would like to create a script/widget for Wordpress that will allow us to add a line of code to the slide-show that will show an Adsense ad after every 4 slides. I do not want overlay, just an add that can be slipped for the end user or clicked on if they desire. I am currently using the standard Wordpress gallery plugin. I look forward to receiving your bids. Many thanks for your interest.