XS4 = inner diameter + .637" or 16.18mm
The XS Model has a bit more depth to the cup than it's shallower counterparts. The shortened down overall length of the LX design, along with the #27 bore (.144 or 3.65mm) combine to make this model more open than most other lead pieces. In this way the XS is unique offering a cup, that while still on the shallow side, gives players that extra bit of room for their lips to vibrate freely without bottoming out. This model is truly versatile, offering a massive sound with lots of playability throughout the entire range, low to high. If you've never really been able to play a shallow cup before, this is the one you may want to try!