There are new vector clips supplied with TIR4 and available for purchase on naturalpoints site.
These are slightly longer and curved. They sit on their own holder so remain in the horizontal when you turn your head.
I will do some tests with TIR4 using old and new style to see if I notice any improvement- some on SIMHQ forums have reported that the new clips improve tracking with TIR3 so they might be worth considering if you are having tracking problems with TIR3.
EDIT- I have tested the two types of clip and have not noticed any real difference. If you are having problems the new type are a cheap way to try improving tracking or see charvels guide on this site on how to set up your system.
http://www.airwarfare.com/guides/tir_setup.htm