[CentOS] Port knocking and DNAT rules

So I found an excellent port knocking tutorial using ONLY iptables rules 
that looks to be among the best I've ever seen. (warning: techno music, 
tough to read screen, you don't need to type it in because I post a link 
to script below)

http://www.youtube.com/watch?v=0zFQocf7C_0

It works fabulously for simply opening a port to a locally managed 
service, but I can't seem to get it to work for a PREROUTING/DNAT rule. 
I've posted the shell script I'm trying to get to work, it should be 
self-documented.

http://chico.benjamindsmith.com/iptables.txt

I've confirmed that the logs correctly show port knocking 2, 3, and 4 in 
/var/log/messages so everything seems to be working golden all the way 
up to the last line. There are no errors reported when I run this 
script. The result that I get is that it acts as though packets are 
being dropped for 15 seconds, then I get connection refused.

What am I doing wrong?