A thought just occured to me:
Rather than finding the specs for the modified servos themselves, check the PWM pinouts for the 4-port EDU PWM's (
page 8)
Indeed, like you said, the 4th pin is a +battery pin - same as the middle pin in a 3-pin PWM connection.
Now, there are a few questions. First, do the two +battery pins supply the same voltage. You can check this out with a multimeter while attaching the multispeed servo to the edubot.
Second, whats the current draw on those modified servos? I'd say just splice that 4th wire to the other +battery, but if the current draw is too big, you run the risk of frying your RC (the breaker description is vague, but it sounds like the built-in breakers only protect the additional battery, not necessarily the PWM ports themselves).
I'd say measure the current draw on the modified servos when attached to the RC - both under straining and free conditions. The safest bet is to attach that 4th wire to an external battery of the appropriate voltage, but if you're feelin' lucky and you tested the current draw, splice it to the center pin of the 3-pin PWM connector.
...we really need one of those electronics people in here