Colonia del Sacramento is an easy ferry trip from Buenos Aires. A very interesting place for a day or two (four would be too much I think). It's a little 'touristy' but nice; much, much nicer than Montevideo.

With four days available Iguazú Falls is an extremely nice possibility.

Another possibility, a little harder to organize and not sure if it can be worked on the same type of award: Easter Island (IPC). Perfect for 1-4 days and definitely fits the bill as an 'experience', provided you aren't looking for a Punta Cana or Papeete style of place.

I could go on and on, but all of them are places you would have to reach from either EZE or SCL, and don't know if that is of any interest to you.