Dave,
just to add to Sandors description above, the "2 servo extender" (top image in Sandors post) also gives you two additional EX ports for connecting up to 2 EX Bus sensors. This effectively makes it a "Y" cable for EX Bus sensors at the same time it gives you 2 traditional servo outputs.
The "4 servo extender" (lower image in Sandors post) does not give you these extra 2 EX Bus ports, instead it gives you 4 traditional servo outputs.