The $55 ($52 or $54 depending on size) price is all 4 fins to clarify.
fins from 2008 and older are different than the newer ones. In 2009 slingshot beefed up the fins and even have wider screws (1/4"). this "beefing them up" thing makes me wonder if slingshot might warranty them too like eric said, but i don't know.
IF you used an older fin on your new setup i would
definitely use some washers! like i said above, the older ones have smaller screws.
knowing that $55 bucks does not sound fun right now, the prospect of spending even more may not fly with you, BUT if you like the idea of going aluminum then try out CDI Fins.
http://cdifins.com/cdifins_fins.html
I switched to these last year and am very happy.
I recommended them in another thread here that you can read more about them.
http://www.wakeworld.com/forum/showthread.php?t=786243