Now you can play this game in 4k

While many modern games support native 4K resolution, it appears that it is a common issue for game developers to incorrectly create their game project at 1920x1080~2560x1440, and be technically stuck with it. As a gamer, if you failed to check if this game supports your native resolution within 2 weeks after purchase, then you are also stuck with it; Steam platform can be cruel sometimes.

Why upscaled graphics can look bad

Windows OS uses a legacy Bi-linear interpolation filter by default. This filter is very efficient in computation usage, but produces one of the worst results, leading many people to believe upscaling is a no.
In an alternative measure, Windows allows you to choose "Forced native rendering". Unfortunately, this is never going to work with game engines.

Why is Bi-linear interpolation bad

A linear interpolation filter takes 2 points, and map a straight line inbetween them, which interpolates extra pixels inbetween.
A Bi-linear works in 2D, by completing 4 linear interpolations in a 2x2 grid, then using these horizontal and vertical lines to further interpolate the center holes in each 2x2 grid, in this case, it is for each 2x2 pixel area for an image.
The problem lays in the "straight line" being interpolated, as most content, especially texts are sharp edges (suddenly jump from a pixel value to a much higher/lower one). After Bi-linear filtering, the edges become mushes.

Will Windows use better filters

Windows OS is always designed with office environments in mind, not gaming. Its default upscaling method prioritizes performance in ancient Intel Pentium/Celeron office machines with a low-DPI monitor, only recently adapting to high-DPI monitor. Changing the filtering method now may impact their business, so probably no.

Performance and latency considerations

It varies in the type of filter and strategy you choose:

• Simple Filters: Filters that doesn't use temporal information (previous frame), only taking one picture, they are very fast, not the best looking, but delivers much better results than a Bi-linear filter
  
• Neural Network Filters: Delivers highly accurate result but can be computing and latency intensive, not decent for compute-demanding game
  
• Filter Chaining: Use 1 filter for upscaling, then feed this result to into other filters for artifact clean-ups
  
• Oversampling: i.e., A maximum 2560x1440 game in 2x upscale would results in 5120x2880, to fit a native 4k display, another filter is used to downscale it
  
• Emulation: To reproduce a correct CRT-monitor render for a specific game, you may use such a filter to reproduce (and upscale) it

In 4k upscaling, you should expect added delays ranging from 1.5 to 6 milliseconds, which is relatively minor compared to other latencies (e.g., mouse-keyboard, Bluetooth, rendering pipelines) for around 20 milliseconds in an acceptable case.
An acceptable delay for competitive shooters or rhythm-based titles are quite tight, but these games, especially non-indie games are not going to just support up to 2560x1440 and call it a day, or they are out of business. For a 2D unity game, latency should not be an issue for you, in fact, your graphic card's fan may not even spin with the right filter chain selected.

Magpie Upscaler - a blessing for gamers

Link: https://github.com/Blinue/Magpie. Magpie is a powerful open-source tool that allows you to customize your filter chain and achieve stunning good results. You can have these features for free:

• Nearest-neighbor and MMPX: for pixel-art graphics and uses very little computing resources, you may see artifacts if the image has non-pixel art overlays
  
• Bicubic and Lanczos: for all-rounder and uses very little computing resources, might be blurrier, but way better than Bi-linear
  
• Anime 4K GAN: for all-rounder and uses some computing resources due to neural network being used, but achieves a pixel prefect result
  
• FSR (FidelityFX Super Resolution): AMD's upscale filter variants, it balances performance and graphics especially in 3D games, but older versions may have edge case artifacts
  
• De-band/De-ring/De-noise: remove artifacts caused by the previous filter in the preset chain
  
• CRT Genom: Uses some computing resources, to correctly render games designed for CRT monitors
  
• NNEDI (Neural Network EDI): One of the best de-interlacing filter for older games that only supports interlaced displays
  
• Auto-oversampling: Magpie would automatically put a bicubic filter to scale a window back to native full screen display. Note that downscaling is reducing information, which doesn't require a very good filter in comparison
  
• In-game overlay toggle: In learning phase, you may try a bunch of filters and evaluate their feasibility
  
• Filter parameter configuration: You can adjust how some filters for to better fit them to your usage
  
• Preset per-app: As stated, different game have different art styles, this allows you to switch inbetween games easily, once configured

Conclusion

This article is kind of a response to the silence on displaying issues from the developers. My research and suggestion may have correctness problems, but the proposed solution has truly conquered this disablement of 4k gaming, thanks to the passionate creator(s) who made it possible.