Miditech MIDIface 4x4

Works perfectly with Windows 8.1 and so far it has even been stable through a usb-hub and a long usb extention cable, which normally is adviced against. I like using this usb midi interface better than using up four usb ports in my computer. My usb ports are needed for other devices like ilok, elicenser, printer, mouse/keyboard etc. Of course this means that you need keyboards with midi out ports, but those exist and are not more expensive, except you often have to buy external power supplies for those keyboards. Anyway, this is a nice midi interface and not too expensive.

Just WORKS - four master keyboards - SUB midi class compliant in windows 10 and linux (ubuntu studio)