Set up a 4K which is 3840x2160 sequence and export as such.
I don't know Mirror Lab or Glitch Lab, but they might already have reduced the image quality, at least they reduced the resolution.
What is the extension of these images.
4K is 3840 X 2160 pixels.
If your media is 4k, your sequence is 4K, and your export settings are 4K, then you will have 4K.
If your display is not 4K, then it will either rescale it or crop it, depending upon the device and its settings.
Ann and Peru Bob perfectly illustrate a nomenclature issue ...
As "4K" is, in a strictly technical term, several things.
There's 4k DCI, which is 4096x2160.
There's UHD, which is also often called 4k, and is 3840x2160.
And at least three cameras I know of do "4k" with their own specific width, running around 4100 plus a few pixels.
But as to your question, if your sequence framesize is X, and you use Set to Framesize to move your files onto that framesize, the resolution will be that of the framesize of the sequence.
Whatever you set for that framesize.