I really like my original Gt4 but just hate how big it is. The Gt4 S is a smaller version of the original. It still has the same deck but has a smaller tank. It’s 23mm in diameter and almost 1 inch shorter. Flavor is the same or a bit better due to a shorter chimney. 3.5ml capacity works fine for me. If you have a Gt4 but hate the size this is the tank for you.