Modern Times is an expansion being sold on Steam as a Downloadable Content Package. This is somewhat unfortunate as you can no longer player with just the original buildings of Tropico 4. I have some mixed feelings towards Modern Times because Tropico has always been a game about a communist country stuck in the past. Now you can modernize your island nation with all sorts of skyscrapers and modern amenities.
The new game includes several new buildings and allows many buildings to be upgraded throughout the game. By far the best new feature is the metro station. This is a very small building and while you are not actually building any real subway system, the building is extremely useful. Basically it acts a teleportation station. Any citizen can walk in one metro station and end up at any other metro station that you have placed. So you need to build two for people to start using it. The more you have the better. Once you start building these, people stop using Garages nearly as much and traffic flow will hardly be a problem any longer. This is an extremely expensive building to build though.
Other buildings include towers that can be setup as mortgage centers and banks. There is a new massive hotel that towers over the previous Hotel Tower. There is an aerodome which supports more tourist traffic. By far the most silly aspect of the game is the inclusion of a space launch center. While you might have to build some really weird series of buildings to get it, it totally removes the realism of the game for this small communist country to build a space shuttle. One event allows you to travel to the moon. Tropico has always had some silliness, but this takes it a bit too far in my opinion.
Good new buildings include, solar power stations, vehicle and electronic manufacturing and upgraded apartments and condominiums. New much larger farms provide much needed food. Overall it is a great expansion, but for me it kills the theme of the original games by quite a large margin. It is certainly worth looking into. Like it or not, Tropico has been modernized.