Well

IRQ stands for Interrupt Request. This is the request every piece of hardware has to do to the processor to get a job done.
If the processor gets simultenaous request from different pieces of hardware, he will handle the lowest IRQ first, that is 1 normally, and so up to mostly 15.

So 4 has to wait 'till the processor has finished the job from 3 and so on...

Hope this helps a bit,

Greetz,

*** EDIT *** Take a look at this one , http://www.webopedia.com/TERM/I/IRQ.html