i ride an 18″ 456 but recently have started to get lower back pain. This has only started since i switched to the 456. I’m guessing that this is due to bike set up and bad riding posture, but is this normally caused by too short a reach or too long a reach. I’m using a blackspire 500 stem so it is short with no rise. Would a longer stem with higher rise help?

too long a reach, I would have thought.

I went from a 19inch Kona to an 18inch to alleviate some back pain, but also trying to touch my toes whilst keeping my back straight, so it pulls the hams/gluts, helped a lot.

Also look for some of the pyriformis stretches on youtube.