As far as I understand, the 4500 shifter can be used either with the 4500 (double ring) or the 4503 (triple ring). My bike has the double ring.
My problem is, my setup seems to be adjusted for a triple ring - it takes two shifts to move the derailleur to shift the chain in either direction. Moreover, the shifter shows three positions for the red indicator. My understanding is that with a double ring, only two of these positions should be in use. But that's not what is happening. When the indicator is to the left, I'm on the large ring. Hit the downshift lever - the indicator moves to the center, the derailleur moves toward the inside, but not enough to shift the chain. Hit the downshift lever again - the indicator moves to the right and now the chain jumps to the small ring. Moving back to the large ring, it's the same process in reverse.
The setup is functional, however I'd very much like to be able to make the shift with a single, rather than a double tap. If you can offer any suggestions as to how to correct this, I will be most grateful.