They will vape at 4.2 for a good half an hour or so. Then you can choose between the standard 3.3 of a normal ego or 3.7 of the Kgo or Riva. I didn't see them until I was already on 3.7v batteries so I didn't see the point of backing down to 3.3. But they would be good if you are testing the waters and want to know exactly what voltage you are vaping on for different attys and carts.
Liberty flights has them. They label them a Riva Variable Voltage