4K 4:2:0 is absolutely nothing like 1080p 4:4:4 upscaled.
4K and 1080p obviously refer to the resolutions of the signals; 3840x2160 and 1920x1080 respectively. Each pixel of this signal should have three components. RGB (Red, Green Blue) is a standard way, but there is another, YCrCb. In YCrCb each pixel has three channels like RGB, but they are Luminance and two Color channels. Ideally these channels are delivered with a full sample of data per channel per pixel. This is not always the case...
Subsampling is the reduction of this data to compress the video signal and require less bandwidth to transmit. 4:4:4, 4:2:2 or 4:2:0 are references to what kind of subsampling is happening. The ratio is described by three numbers each describing the number of samples in a pixel area X pixels wide and 2 pixels tall. The first number describes the number of pixels wide we are referring to, in this case it is always 4. The next two numbers describe the number of samples taken. The first of these is the number of samples in the first row of pixels. The second is the number of
differences between the first and second row of pixels. So it is (number of pixels wide)

samples in first row)

differences between first and second row)
Luminance is always transmitted at a full sampling rate and is never compressed.
So for 4:4:4 we have an area of 4x2 pixels. There are 4 samples taken for the first row, and the second row has 4 different pixels below the first row. This is describing uncompressed video since all eight pixels are unique.
4:2:2 again describes a region of 4x2 pixels. There are 2 unique chroma samples in the first row, and 2 changes between the first and second row. This is describing a halving of the horizontal chroma resolution since we only end up with four unique color samples for this 4x2 region.
4:2:0 also describes a region of 4x2. There are 2 unique chroma samples in the first row of pixels and no changes between the first and second row. This leaves us with 2 unique chroma samples in the 4x2 region so this is describing a halving of the vertical
and horizonal color information.
Wikipedia is does a good job with pictures.
TLDR: 4K and 1080p always transmit the resolution video they are describing, however, the color information can be compressed (luminance is not). 4:4:4 is uncompressed video. 4:2:0 is compressed video (has 1/4 the color information).