In my world , the entire population is concentrated into 5000 square mile modern cities. The population density in the cities is around 4 million per square mile , with mile-tall skyscraper's that serve as capital buildings. These cities have the technological advancement equal to that of Tokyo.

My question is , what major issues would be involved in maintaining a city of such population density?

  • $\begingroup$ You're kind of confusing in the question body - how can you have a single 5 thousand square mile cities (more than one?) Are you talking about lots of cities with 10 billion people in them or one city with 10 billion people in them? The rest of your question also uses cities, while your question title uses a single city. Make up your mind please. $\endgroup$ – Aify Sep 27 '16 at 18:21
  • 5
    $\begingroup$ There's likely to be a huge list of issues. Off the top of my head, transportation, food, health care, crime, and government will all be vastly different in a city of that size. Breaking this question down into individual concentration areas would make it much more manageable. $\endgroup$ – ckersch Sep 27 '16 at 18:34
  • $\begingroup$ This question is related and may be of use: worldbuilding.stackexchange.com/questions/12113/… $\endgroup$ – James Sep 27 '16 at 19:52

Sickness & Pandemics

Any place populated that densely is prone to get swiped away comparatively quickly by some major outbreak of something that cannot be easily contained1.
Take a look at Tokyo, even though almost any japanese that's got a simple flue or cold will wear a surgical mask (or whatever they are called) to keep the infection from spreading, it still manages to infect new people easily2.

1read: Really Anything At All
2Check out this site on worldwide deathrates due to different causes; Japan has a huge toll on deaths due to influenza & pneumonia