Yes, the BEC on the Castle speed controllers is designed to handle the voltage from 12 NiCd or NiMH cells connected in series, not "12 cells" period.

Actually, the voltage from a 4S LiPoly is approximately equivalent to 14 NiMH cells. Even with a 4S, you are exceeding the BEC's capacity, and you should disable it by pulling the center pin from the receiver connection to the ESC, and taping it off.

The problem is that the BEC regulates the voltage down by burning off the excess as heat. When the difference between the main battery voltage and the 5V needed for the receiver gets too large, the BEC gets overloaded. It may be burned out even now, and connecting receivers will only continue to burn them out too. At the very least, disable the BEC and use a receiver pack or a UBEC as recommended above.

A UBEC is a different kind of regulator, called a switching regulator. The BEC in most ESCs is what's called a linear regulator. The main difference is the UBEC regulates voltage by switching it on and off very quickly, and as such can handle much higher loads and much higher voltage differences. Why don't ESCs come with switching regulators? They're expensive; they'd add $60 to the cost of each and every receiver across the board, which would put the receivers right out of most people's price range.