Special Class for Last Post

Hey guys,

I’m trying to make the last post in a category to be of a certain class (CSS). What am I trying to do?

http://alpha.rosepizzulo.com/

If you look at the four thumbnails going across, I essentially need the LAST THUMBNAIL not to have any margin-right. Right now, I fixed it by changing the HTML in the farthest post to the right to have

<p class="try">

How would I be able to do this automatically (this is for a client) for, essentially, every 4th post in that loop?

Thanks,

James