I am having the same issue. I do not have any themes or apps installed at this point in time. The 4th thumbnail image is showing up as the main product image. I have tried re-organizing my images, but no matter what image I use as the "large" main image, the same image always shows up on the product page as the main one. Has anyone found a fix for this?
I was experiencing the same issue.
It seems that shopify shows the variant image that is selected, instead of the featured image.
Try follow these steps, to see if it helps you out: https://shopify.dev/tutorials/customize-theme-show-featured-image-on-product-page