I created a number of hard start VO2max workouts a while back.
When creating the workouts, it took trial and error to dial in the right intensity (and progression) that felt right for me. So it may take you some similar exploration to determine what works for you.
I’d also say - going into 6x4 right away is hard. If I were in you’re shoes I’d start with something easier to begin with. Ex 6x2.5.