Where do you plan to watch the 4K version of Spider Man? You might get the free upgrade to 4K depending on how you redeemed the HD version. In my case, many of my previous movie purchases or redemptions automatically upgraded to 4K but only if viewed on my Apple TV 4K. That was a major perk for me and made the purchase of the Apple TV 4K even more of a bargain.
Have you tried actually playing them? HD movies ported in from MA show 4K on the overview page, but currently only play in 1080p HDR (you can tell by pulling down the info sheet on the ATV while the movie is playing).Many of my movies that were purchased and redeemed through Ultraviolet and now reside on Movies Anywhere have become 4k automatically. I even had some SD versions that magically became 4K on iTunes.