Which of four switches lights the bulb

A closed room contains one bulb. Outside are four switches, only one of which controls it, and you cannot see the bulb from the switches. What is the minimum number of times you need to go into the room to know which switch is the right one?

$1$. Use heat as a second bit of information: turn switches $1$ and $2$ on for a while, then turn $1$ off and $3$ on, and enter. On and warm means $2$, on and cold means $3$, off and warm means $1$, off and cold means $4$.