If they are not explicit and not explaining this even to the 1st recruiting filter... it means the only way it happens is if you get in the company, do 8 hours for a while, prove your worth, then switch to 4-6 hours. It's a silly dance and takes time.
I did see it once, and even then they just wink-wink approved 6 hours with a reduced pay but legally you were on the hook for 8 hours (at the reduced rate). Flexibility varies...