4ml + juice capacity might sell me on it. I hate to go out for the day without enough juice in the tank to last a while. I usually carry a zipper case with 10ml juice, a spare 18650, and a mini screwdriver. When I do that with my old KF Lite v2 I seldom have to use the backup juice unless I get caught out longer than expected, but it's nice to have in the vehicle if needed. I do like the 24mm tanks that offer the same juice capacity in a lower profile. That's what sold me on the DoggyStyle and the EhPro True.
Juice feed control is a plus for leak free tank filling. The Coppervape Skyline fills with no leaks or juice in the mouth afterward if you close the JFC before filling. It's a great design.