I have had a TrackIR 4 since just after they first came out and could not fly without it now but after several years heavy use (and I mean several hours a day, week in and week out as I am stuck at home 24/7 and do a lot of flying, beta testing and paint/model testing) it has started to go myopic in one 'eye'!
It works fine from left to right but the up and down range has been slowly reducing over time as has the distance it can see clearly (I have to drag the monitor forward now when I fly as it loses track if I sit too far away from it). I have even had it all apart and washed off all the emitters and receivers with Isopropyl but it's no better. I had some great assistance from the tech support guys at Natural Point and they were as confused as I was with regard to this problem. My personal feeling is that one (or more) of the LED IR emitters or receivers is getting 'old' and starting to lose it's effectiveness.
Bottom line is that I am now in the market for a replacement and even though I can get a TIR4 for less money (still lots around) I will be getting a TIR5 as the extra viewing range will be beneficial to me and the 5 is supposed to be much smoother and more accurate in it's tracking abilities.
Unfortunately it looks like I will have to resort to Ebay as I can only use PayPal to make the purchase and the best price I have found so far is around $150 USD.