I have an old iPod that I doubt can play more than SD, but I bought 4K for my newer items. Did Apple include the SD version in it? The iTunes store on iOS 6 doesn't work anymore for viewing movie pages, so I have to try and download the SD version of the movie on iTunes and sync it over.
According to Apple's site about downloading or streaming movies:
After you buy a movie or TV show from the iTunes Store, you can stream it on the device that you bought it on or on any of your other devices, as long as you're signed in with the same Apple ID.
Videos stream and download at the highest-quality version available based on the device that you're using, your device's settings, and your Internet speed.
The SD version won’t have iTunes Extras, 4K, HDR, Dolby Vision, or Dolby Atmos formats, even if you play it later on a device that supports those features and formats.
According to that, I would say it will stream to your older device but the SD version is not automatically downloaded along with the HD version.