That Air Ride will fit an 04 chopper. I am in the process of installing that same Air Ride system on my 03 chopper. The shocks bolt in the original location with no problem but there is a couple of things you may have to deal with.
I don't believe the 03's or 04's are pre-wired for air ride like the 05's and newer. The other problem is where to mount the compressor. I found the perfect spot right behind the transmission.
There is an excellent acticle with pics @ hotbikeweb.com
In the search box type '2006 mastiff air ride'
FYI, Shovelheadkicker (on ebay) sells used set-ups regularly for $400 to $500.