I have, in the past used a series resistor and a zener to ground to ensure +4.2 for a high and a diode parrallel with the resistor to ensure the low (make sure you use a regular zener that kill negative input voltages). This arrangment may, in case of severe negative spikes on the input, lead to a burnt diode, zener and chip. The real problem is not the voltage levels, but the noise typically present on such lines. To protect against ground noise and negative undershoot the optocoupler is much preferrable. Make sure the optocoupler input has a reverse diaode across the input and a series resistor.

Have fun,

Erik