Actually, I'm not sure the pixel density is higher on those... here is a 4096x2160 diagnostic monitor:
http://www.eizo.com/global/press/releases/htmls/RadiForceRX8...
and it is 36.4" diagonal.
http://www.eizo.com/global/press/releases/htmls/RadiForceRX8...
and it is 36.4" diagonal.
[1] http://www.necdisplay.com/p/medical-diagnostic-displays/md21...
[2] http://www.barco.com/en/Products-Solutions/Displays-monitors...