Dennis,
The CC Phoenix ESC's use what is know as a linear BEC for powering the servos. What this means is that to get the voltage down in a range so that it doesn't fry the receiver, servos, and in our case the timer, the battery voltage is dropped across some type of transistor like object. The implication of this is that this transistor is "eating" the power of the higher votage, specifically this power equals just the current out of the BEC into the receiver/servos * the voltage drop.
For a 3 cell lipo the voltage drop is about 7 Volts or so (12V-4.8V). I believe typical current requirements for rc setups is somewhere in the range of 0.3 to 0.5 Amps (40 sized 4 channel ship). So this would say the BEC would have to "eat" 7V*0,5A=3.5 watts of power. This is a hefty amount of heat to dissipate, which is also why they tell you how many servo's you can attach. If you had a 4 cell lipo, the voltage drop would be ~16V-4.8V) or 11V and now the heat to dissipate 5.5 watts. They are telling you the BEC can't handle this much heat.
Now we just need enough current for our timer (very tiny) so I doubt that we would hurt the BEC, even with 4 cells, but I would certainly contact Castle Creations first----but on the other hand, almost everyone I think is doing it.
By the way, I just posted a review of the JMP-2 and Ztron CLT3 timers in the review section, for those who may be interested. I mention this mainly because I often forget to look down that far on the Forum Topics.