Quick background: there are two kinds of 4K movies available on our Movie Store today. There are 4K Ultra HD movies, which are 4K resolution, 10-bit color, BT.709 colorspace, and standard dynamic range. And, there are 4K HDR movies which are 4K resolution, 10-bit color, BT.2020 colorspace, and high dynamic range.
Many 4K movies are available in both of these formats, but not all. There are a fair number of 4K Ultra HD movies that have no 4K HDR version. There are a few 4K HDR movies that have no 4K Ultra HD version. If a movie is available in both 4K formats, your purchase includes both formats. 4K purchases also include the lower-resolution versions like Blu-ray quality.
With that background, let's look at your situation. Strato can down-scale 4K Ultra HD movies for display on HD displays, so you could certainly purchase 4K Ultra HD movies and let the Strato down-scale them for you.
Strato cannot, however, convert 4K HDR movies for display on standard dynamic range displays, including HD displays. If you were to buy a 4K HDR movie, you would not be able to view the HDR version, but you could download and view the 4K Ultra HD or Blu-ray quality version if available.