Here's a link to a tutorial on 555 timer circuits. How do you want to start the timer? Push a button then the fan runs for 4 seconds then turns off until the next push? Or, waits a period of time then turns on again for 4 seconds? If you can't figure out what you need post your problem in more detail and someone will point you the right way. If you google "555 timer circuits" you'll get lots of links. 555 timers are very well documented on the net.