Schools? Amusement Parks?
Schools, big cities, and bio-engineered Chicken Pox, perhaps. It's easy enough to get, extremely commonplace, and widely considered to be harmless.
But if we look at it, the terrorists would be utter fools to do this sort of thing. Not only would they lose any claim to humane treatment (the only thing holding us back now), but they would ensure that we'd get pissed off enough to spare no effort in hunting them down for real.
Beyond that, such an attack would be meaningless in a strategic sense. Even 4 million people is an insignificant percentage of the population; and I think they've probably grossly underestimated the capabilities of the public health system in stopping the spread of the disease, if such exists.
Schools? Amusement Parks?
Any largish city will do, if your tool is a nuke. Don't forget about the 4 million total threatened deaths.