I can't see apple releasing a new 4" phone in the near future, but trends could mean a smaller phone than 4.7 could come later. I think the 6+ may end up being an aberration, but only sales and time will tell. Much like the 17" MBP, It could be a temporary thing.
Making calls is only one feature of an iPhone, and, for me, the least used. Instead, it is mostly a portable computer and internet access device. I dreamed of this years ago. Today, it is reality.