1. go to the fourth folder on your computer where you store your pictures,
2. pick the fourth picture in that folder,
3. explain the picture,
4. tag four people to do the same
Okay, this picture is ~/Pictures/Misc/070907_mouse.jpg
As for an explanation... well, first go back and watch the 2007 Doctor Who episode The Family Of Blood. And then have a look at

I'm sorry, I'm so sorry.
Oh, people to tag. Hmmm.