I have the same problem with the lenses on my -4. After 13 years in the air, mine are a bit cloudy, and have many tiny stress cracks within the thickness. I just ordered and received new lenses that I will install someday soon. If your lenses are cloudy solely due to surface erosion, you may be able to used an auto headlight polishing kit to restore them. I tried it on my car this past summer, but determined, after much sanding, that my lenses were too pitted to ever polish to an acceptable level of clarity. I have decided to buy new lenses for the car, too.
Good luck,