Good suggestion, it would be about Arg$40 each way to their station in Puerto Madero. Here is the link http://www.tiendaleon.com.ar/home/home.asp

You could make it into the city, have lunch and back within your four hour time frame. There are many restaurants to choose from in Puerto Madero so even allowing for one hour travel time each way (should not be that long) would give you a couple of hours to enjoy the city.

Suerte
SAM