Google announced this week that movies that were purchased in the Google Play Store will be automatically upgraded to 4k when it is available for that movie. It does not matter if it was purchased in SD or HD.
Originally the price for a 4K movie would cost in the $25-35 range, while HD and SD would be much cheaper. As per all new technologies, it was expected to have a higher price point for 4K movies when most people don’t have a 4K TV to support it. Now that the market has adjusted a little, Google announced they will be lowering the price for new 4K movies as well. They anticipate the costs to be between $12 and $20.
Because most people don’t own 4K Screens, it will look better than SD, but roughly the same quality as HD. Either way, Google is looking out for the future and allowing people to access content they already paid for. Thanks Google!