Actually, the outputs from the ERR boards (really the R4LC) supply a half-wave negative going signal when the lights are on in command mode. However, in conventional mode, they output a full-wave AC signal, which I why I normally include a diode in the lighting circuit.
If you put all four in series, you'd need about 10 volts effective power to light them. In that case, I'd probably use a 150 ohm resistor.
