Re: @ Loyal Commenter - not only, but also......
I am not so sure that four billion light years away in this setting doesn't mean distance to where the black holes were when they emi... didn't emit any light. In astronomy it makes just as much sense to speak of the light travel time distance as to talk about comoving distance. After all that is what you are actually observing, an object that is x billion light years away. The fact that it has moved a lot in the time light took to reach us and that space itself has expanded doesn't change the fact that we are observing something now that was x billion light years away then. If we suddenly should talk about where it is now we might as well talk about that it might not even exist either. Now I had said what I needed to say, under follows a rant.
It bugs me that they have buggered this up in the observable universe definition. It is in the name for <deity here>'s sake, observable. To talk about the comoving distance then is just mind-bogglingly stupid. We are not observing a 46 billion light year universe, we are observing a 14 billion light year universe. Where these things are now doesn't matter as we are not observing them as they are now anyway. If anything we should talk about the angular diameter distance which does factor in the expansion of space, but instead of projecting what we observe in the past into how it is now it tracks backwards to how far away we were at the time light where sent out. So it would make more sense to say that the observable universe is about three billion light years in every direction. However, I do think the muppets, fiddlers and semanticeers have settled this already I am afraid, so they can argue with people that don't care at parties about "No that is a misconception, the observable universe is 45,7 billion light years in radius, you see..." after that they will go on at lengths about how glass is either solid liquid or a liquid solid.
The thing is that when you talk about distance and you have several ways to measure it you have to actually tell us what metric you use. Once you do that any argument falls flat since it is in the clear what you mean. For instance measuring distances on Earth and you say "in a straight line" it usually means staying on the surface and not cutting through planet. But you can still go to parties and say "but a straight line doesn't curve so the distance between New York and Los Angles is really, yada yada".
As for your orbiting itself or each other comment, the sentence in the article makes perfect sense.