Q: Do anyone know What type of material is the lens cover to the rear cover of the iphone 4S and how is affixed to this?

Yes, it is clear plastic. Both the clear plastic lens and the black housing are held in place by simple soft adhesive. The lens can be simply be pushed out of the housing with a Q-Tip or wooden toothpick and be replaced by pushing new lens in.

They are wrong, it is simple clear plastic. Anyway, what does it matter. Sure plastic can be damaged easier than glass, but even glass can break or scratch. The lens on most cameras today are made completely from plastic. This lens is part of the back case, and not the rear camera which is below.

http://support.apple.com/kb/HT3226#iphones

Don’t use window cleaners, household cleaners, aerosol sprays, solvents, alcohol, ammonia, or abrasives to clean iPhone.

As BT says, not a good idea. The adhesive under the lens will dissolve with alcohol. To clean any part of iPhone exterior just use clean dry lint free cloth.