That's a quad solution: four dies on one chip. Each one is 21.5 x 21.5 mm and should have 25 GH/s, but it gives us the same η value as one 43 x 43 mm die outputting 100 GH/s..
All the multi-die packages I've seen leave a few mm of space between dies. So the actual die size is likely to be under 20x20