Cindy,
The "-4" is part of the model number that shows a design change. The model # would be 66-4.
If you go to ISMACS at: {
http://www.ismacs.net/singer_sewing_...ist/index.html }, click on the ( Classes 1 to 99 ) section then scroll down to the 66s you can read about the changes and features.
Go here: {
http://www.ismacs.net/singer_sewing_...-database.html } to date the machine.
"Side clamp feet" is just another way to describe "Low Shank" feet.
Joe