Wide voltage supply for PIC microcontroller
When Vin goes below your minimum Vin you are going to have your output drop. I have seen many times that people think it will be a small drop and it ends up dropping very quickly when the Vin goes to low.

If you want something that give you 5 out from 4.5 to 18V in, then you should just pick a simple Buck-Boost DC-DC converter. They will be able to handle from 2V to higher than 18 without a problem. By grace of being a buck boost, they can step your voltage either direction and it should make your life quite easy.