i have trackir 4.
its awsome! takes a little work to get the profile set up for you personally. alot of into the game to test out of the game to correct ect ect, but once its done and your happy with the tweaking youll think its the greatest thing since sliced bread or a tater shooter!
i stopped using the track clip pro, went back to using the hat. the clip is too touchy and very difficult to get the settings right for, the hat on the other hand gives you all the same movement but is much easier to set up and tweak your profile with.
just remember if you get the trackir 4 make sure the vector and trueview boxes are checked!!! and leave yourself a good sized dead band on all axises, this will help you recenter when your shooting.
good luck and happy hunting