I suspect the C line is going to be the new 4" line. This is what I expect to see this fall
IPhone 6C (5S internals with plastic shell) -$99
IPhone 6 4.7 - $199
iPhone 6 5.5 - $299
So the 4" will get updated regularly, but it'll always be a year behind with a cheaper shell. A bit disappointing but better than nothing. If this happens, I'm still sticking with 4". Being a year behind isn't a big deal anymore now that Apple CPUs are more than fast enough and I can get used to the 5C design.
They probably won't release 3 new phones.
It will be:
iPhone 5c - $0
iPhone 5s - $99
iPhone 6 4.7 - $199
iPhone 6 5.5 - $299