I want to concatenate eight png-pictures each in the size of 21600px x 21600px in the form of 4 x 2 pictures using any program in windows. Trying ImageMagick or GraphicsMagick resulted in an error (due to a limitation of 65500px in any dimensions).  

I also tried it with gimp, but it's too much for my computer (and gimp). I prefer a light program or even a command line program. Do you have any recommendations?