What size Exo Terra Repti Glo 10.0 bulb is best for a 4ft vivarium?

Although it depends some about how much heat you need, I'd go with the 150, or the 250 if you aren't using another heat source. I use a 150 with two under-tank pads in my 60 gallon tall, and I can keep the heat up to around 90 if I really need to.