There is one for vb3: www.vbulletin.org/forum/showthread.php?t=119746 I think it might work for vb4 but the template edit (in the instructions in the first post) needs to be updated for vb4.
A lot of people in that thread were asking for a vBulletin 4 version, it makes me wonder why it hasn't been done already by someone.
How could that mod be converted/updated to work with vB 4?