People can postulate and guess. Sometimes they're right and some times they're wrong.
Just look at the rumors of apple producing some sort of voice controlled television. For the past few years such a product was imminent. Now that rumor is largely dead.
My point is we don't know, though it seems like a valid rumor but its who knows.
I'm skeptical. It is very possible that they will make a 4-inch iPhone 6, sell it at $99, and do away with the 6 and 6 Plus. There is also a possibility that they will upgrade the 5C with Touch ID and A7. We'll just have to wait and see.
My guess (if I had to guess) would be that the 5S will be the sole 4" phone in this fall's lineup, and no new 4" model until next year. But it's just a guess...