Which of four switches lights the hidden bulb

Outside a closed room are four switches, all off. Exactly one of them operates a light bulb inside the room, and from outside you cannot see the bulb. What is the minimum number of times you must enter the room to be sure which switch controls it?

Once. Turn switches $1$ and $2$ on and wait several minutes, then turn $2$ off and turn $3$ on, and go in. The bulb's state (on or off) together with its temperature (hot or cold) gives four combinations: on and hot means $1$, off and hot means $2$, on and cold means $3$, off and cold means $4$.