Four Switches, One Light Bulb

There is a single light bulb in a closed room. Outside the room are four switches -- exactly one of them controls the bulb. All four switches start in the off position. You can flip any combination of switches on or off as many times as you like before entering the room. Once you enter the room, you can look at the bulb and touch it (to feel its temperature). After entering, you cannot go back and change the switches. What is the minimum number of times you need to enter the room to determine with certainty which switch controls the light bulb?