I just had a scope with dr Shen (k-pouch) I did exactly what you described, 4 oz miralax with Gatorade zero. They said I could drink something up to 2 hours before scope. I tend to get dehydrated, so that was good news for me.
I drank the miralax around 5 pm and I was done emptying by 8 pm. I also always write down exactly what I did—since I have a scope every year this helps me remember.