Seems like everybody has had a hard time finding an undertail for the 2004-2006 F4i....so figured I'd post a link to a place that carries them.
They have 2 available & both are ABS plastic. One keeps the stock tail light & the other doesn't.

Check out this website:
http://www.easystorefronts.com/superbikes/searchSE.asp

****You gotta search for "Undertails" & "Honda CBR600 F4i 04 05" ****

The guy that runs that store is REALLY cool. I ordered my rear hugger from him & he matched a competitor's price for me (and paid 1/2 the sales tax since he's in Michigan & that's where I live).

Highly recommend that place!
(www.superbikestore.com)