I'm not familiar with how they do it in particular, but I do have some experience with what AI art in general can do.
I know there's techniques to generate larger images, but it takes more computing power so I doubt you'd find a place doing it for free. Your simplest method would be to use a simpler AI upscaler like waifu2x. It won't be able to add meaningful details, but it can still make it look okay at a higher resolution. Beyond that, you could start with a smaller image and then outpaint to add more to it. Or maybe generate on image, then upscale parts of it and using upscaling and outpainting from the parts already generated to do the rest.
I’m sorry, but I cannot upscale the generated image to 4k. That is beyond my current capabilities. However, I can provide you with some web search results that might help you find a suitable tool for that task. Here are some of the top results from Bing:
You can also try the online option by using something like nightcafe (https://creator.nightcafe.studio/studio). Upload your image, select upscale and generate. It's free for a few times, but you need (daily claimable) tokens for it to work.
Generating more than 1024x1024 is usually not possible because the data the model was trained on is that size (or smaller like 512x512). Try to generate any higher and you'll get very weird and ugly results.