The GPU isn't powerful enough to utilize all the available VRAM, so it needs more power (via CFX) to utilize all 4GB. More like the GTX 770, which can't utilize all 4GB by itself.
I don't think the 270X 4GB needs to be paired to use all of its installed VRAM. I'd like to see where that came from. Maybe I'm missing out on something.
R9 280 3GB, you can't game with 2 monitors, as you need 1 or 3 for surround, and 280 will anyways be better than 270X, as 270X can't utilize 4GB VRAM unless in CFX with another 270X.