I think from the sheer experience, TrackIR 4 equals the #5, so no need to upgrade. I have the "5" though.
But I can recommend the combination of EZCA and TrackIR, giving you an easy and always fitting setup for viewpoints of all kind while also enabling some nice effects to avoid the rather static impression of flight sim cockpit.

So TrackIR "pure" is fine, but with the Naturaltrack software alone it's a pain in the a.. to get around some sim limitations for example.
Also, setting up the viewpoints via EZCA is much more comfortable and better fitting than any pre-defined viewpoint which can only be accessed by pressing "A".
I'm e. g. enjoying it very much in planes like the 727 and 707, really finding my spot to operate the FE panel.


Quote:

2-Head movement , is so sensitive and I cannot adjust it.

Oh there's much to adjust, but I'm working with a close to default profile here. I think that Bojote offered his as a download, maybe it helps you, Sanal.
http://forum.avsim.net/topic/326831-trackir-and-fsx-wow/page__view__findpost__p__1927215