What 4K means in pixels
On monitors and TVs, 4K means Ultra HD: 3840 pixels across and 2160 down, a 16:9 frame of about 8.3 million pixels. Cinema has a slightly wider 4096×2160 standard that is also called 4K, but screens, wallpapers and video editors almost always mean 3840×2160. That frame is exactly twice the width and height of 1920×1080, so 1080p doubled on each side lands on 4K, while 720p (1280×720) would need a factor of three.
Because 4K is a 16:9 shape, an image in any other ratio needs a decision first. Either it fits inside the frame, keeping the whole picture with bars at the sides or top, or it fills the frame and loses a strip to cropping. That choice changes how many pixels you actually need.
Upscale image to 4K: which source sizes get there at 2x or 4x
Two rules decide the output. Before processing, Basic, Photo and Screenshot modes scale an image down in your browser if it passes about 6 megapixels for 2x or 2.25 for 4x, and Anime mode does the same above about 6 megapixels at either scale. The result is then exactly 2x or 4x the processed size. For 16:9 sources, that works out as follows.
A 960×540 source at 4x gives 3840×2160 exactly, in any mode. A 1280×720 source at 4x gives 5120×2880, which you scale down to 3840×2160 in any editor. A 1920×1080 source (2.07 megapixels) is under every ceiling, so in any mode 2x gives exactly 3840×2160 and 4x gives 7680×4320, which is 8K. A 2560×1440 source goes in whole at 2x and returns 5120×2880. Below 960×540, for example at 640×360, even 4x stops at 2560×1440.
So to upscale image to 4K, choose 2x for a 16:9 source of 1920×1080 or more, at 1 credit, and 4x for sources from 960×540 up to that size, at 2 credits. The mode only changes how the picture looks, not whether it reaches 4K.
Other shapes: 4:3, 3:2, square and portrait
A 4:3 picture fits a 4K screen at 2880×2160 with bars at the sides, or fills it at 3840×2880 and loses 720 rows to the crop. A 1440×1080 source reaches the fitted size exactly at 2x, because at about 1.56 megapixels it is under the ceiling and keeps its full size. A 3:2 camera frame fits at 3240×2160 or fills at 3840×2560, losing 400 rows. A square image fits at 2160×2160.
Portrait images turn the frame on its side: 2160×3840, the shape of vertical video and of 4K monitors rotated to portrait. The maths mirrors the landscape case, so 540×960 lands on it exactly at 4x, while a 1080×1920 source needs 4x in Basic, Photo or Screenshot (about 3792×6744) or just 2x in Anime mode (exactly 2160×3840).
Upscale image to 4K for wallpapers on monitors and TVs
A 4K monitor or TV has 3840×2160 physical pixels, and a wallpaper covers all of them even when the system scales text and windows to look like 1080p. A 1920×1080 wallpaper is therefore stretched 2x by the operating system, and on a large panel that softness is often why people search for “upscale image to 4K” in the first place. A model-based upscale gives crisper edges and cleaner flat areas than that plain stretch.
Use the fill setting only when the image is already 16:9; otherwise crop it yourself, so you decide what is cut. TVs use the same 3840×2160 grid, but a TV's built-in photo viewer may rescale images its own way, so check the result on the set. One handy coincidence: the 5120×2880 that a 1280×720 source produces at 4x is exactly the resolution of 5K displays, so it needs no resizing there.
Printing at 4K: about 12.8 inches wide at 300 DPI
At 300 DPI, the usual standard for prints viewed up close, 3840 pixels cover about 12.8 inches (32.5 cm) and 2160 pixels about 7.2 inches (18.3 cm). At 150 DPI, which works for posters seen from a few steps away, the same file prints at about 25.6 × 14.4 inches.
For print, keep the pixels you already have. The 4x result of a 1080p image in Basic or Photo mode, about 6744×3792, prints roughly 22.5 × 12.6 inches at 300 DPI, so use that file rather than a copy scaled down to exactly 4K. Fine texture in any upscaled print, such as hair, foliage or fabric, is the model's rendering rather than recorded detail, and that shows more at arm's length than on a wall.
When a source is too small for a good 4K result
With a tiny source, no setting will upscale image to 4K in a single run: a 320×180 thumbnail at 4x ends at 1280×720, and a 480×270 image at 1920×1080. At 4x every source pixel becomes a block of 16, so the smaller the source, the more of the final frame is the model's inference rather than recorded information. No scale undoes motion blur or brings back detail that heavy compression destroyed or the camera never captured.
Runs can be chained: download the result, upload it again and run a second pass. The 1920×1080 file made from that 480×270 image reaches exactly 3840×2160 at 2x in Anime mode. Each pass builds on the previous guesses, though, so textures can turn smooth and painterly, and chaining suits flat graphics better than photos. First look for a larger original, a re-export or a re-render, or use the image at a smaller size.
Cropping and resizing to exactly 3840×2160
The upscaler returns 2x or 4x of the processed size, not a fixed frame, so to upscale image to 4K exactly, finish in an editor. Crop to 16:9 with a fixed-ratio crop, preferably before uploading so the processing ceiling is spent on what you keep; upscale past 3840×2160; then resize down to exactly 3840×2160 with the aspect ratio locked.
Scaling down from a larger result is safe. Stretching a result that came out slightly short, such as 3200×1800 from a 1600×900 source at 2x, up to 3840×2160 adds softness, so rerun at 4x instead, and never stretch width and height by different amounts. Save the final file as PNG if it contains text or flat graphics, or as a high-quality JPG for photos where file size matters.