Post a link to the datasheet for your stepper motor.
When you say "jumps 4 steps" do you mean 4 full steps or four micro-steps.
If you think you have it set for 8x micro-stepping and it is jumping 4 micro-steps my guess is that you have really set it for 2x micro-stepping.
...R
Stepper Motor Basics
Simple Stepper Code
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