you need a temperature gage. that is the only way of knowing what basking light you will need. for little guys, you dont need much heat. 3-4 inches im assuming is about 2-3 months old. a regular 50 or 60 watt, appropriately placed should be ok. but again it all depends on what kind of heat the bulb will create.
Is it the tube UVB? I believe you have to replace it about 8 every months for a repti sun and every 6 months for a repti glo...then you need a basking light. A 40-60 watt will work, depends on your house temps. I've a 60 watt which keeps temps in the low 80s. You will only need to change the basking light as your cham gets older and you can raise the basking temp