Yes you can crossfire a 4890 from one vendor with any other vendor. Just make sure they have the same clocks speeds or manually set their speeds. If you don't, you can get some pretty interesting behavior.
Reference meand it's the original design from AMD, with the blower type cooler and original PCB, just like all the cards when they're newly released, when they all just look the same no matter what brand they are and just slap a sticker with the logo on it.
All those dual fan multiple heatpipe heatsink cards are non-reference designs, and not only the cooler is modified but often also the PCB itself, that's why some cards have a little different sizes from brand to brand, and also why some of then OC better than others.