All my E`s are .105 and dropping to D is fine (both rounds & flats), but I`ve tried dropping to C and for me the string just becomes too floppy for my liking. I`ve only done 1 song that really requires a Low C and if i really HAD to perform it anymore I`d buy a 5.
I had a 5-string Warwick years back and tried for a long time and couldn't get on with it.
I'm happy with a 4-string.