In the process of thinking about a 4th garage ... I kind of wonder if Disney ever though about (in modern era) about creating a standalone parking complex unattached to any park/hotel/complex. While it wouldn't necessarily be need everyday single day, but between the increased need for extra holiday/summer parking at MK and Epcot, overflow for hotels, special events, Cast parking being redirected for those events, plus Disney Springs and the parking crunch DHS may experience once Star Wars opens ... its not unreasonable to think that there may be more days than not parking is being redirected somewhere on property soon.
A modern version of the "Transportation and Ticket Center" might be cheaper/more effective than expanding their flat lots 10-15% and a 4th DS garage.