I thought you were just being a smartass with your response, but if you're not then I can kind of see your angle...
No, that's not what it referenced. It was in regard to the number of innocent individuals killed. The attack had generated so little interest because "only four" had died, these attacks have become so normalized in our society that it isn't given a second thought. We hear it on the news and think "oh, again? That sucks". The post of mine that best expressed my reason for posting the photo went ignored, perhaps because it exposes a humanistic purpose to the motive rather than pure villainy:
That individual woke up, put on his pants, socks, and shirt, and likely expected to have a nice morning shopping. That's how his day ended. Why?
One of four. Only four.