For what it's worth...
If you can find a trackIR 4 for cheaper, go for it. I've tried both the 4 and 5, and I can't tell any appreciable difference between the two. I bought my trackIR 4 for just over half the price of the 5.
Also, don't feel like you need to shell out extra for the ProClip. I bought one, but ended up using the hat clip as I didn't like the wire...and often, when turning to the left, my big fat head would block the clip from the camera.
I do highly recommend the trackIR, especially if you have other games that support it. It really is a great immersion booster.