In fact since cleartype effectively triples resolution I guess you get quality of 12K display?
If you used subpixel rendering to render lines of fractional pixel thickness, it would just appear as a single-pixel line of the wrong color. The same happens if you render line thicknesses up to 1 1/3 pixels wide, but offset to not match any pixel boundary.
At the same time, subpixel antialiasing can only be used with a specific subpixel layout, in the direction sof the subpixels. For regular desktop LCD panels, this means smoothing of vertical lines only, making characters like "o" look funky and uneven.
With that in mind, a 4k panel with subpixel antialiasing does not look anything like a 6k panel with greyscale antialiasing ("super expensive mac screen" is not relevant, apart from display technology differences not related to resolution or aliasing).
Only horizontally (is 4K a double or a quadruple resolution of 1080p?), and only if you don't care about color (saturation, hue).