Well I took that to mean as hard as (I could go for 4 minutes) rather than (as hard as I could go) for 4 minutes, if the distinction makes sense. I did also hold back a little on the first 2 knowing I had 4 more to do, but the rest were as hard as I could manage, mentally anyway. It was also two sets of 3x4 with 10 minutes rest between. It worked out ok, and I trust him.
It is about pushing yourself as hard as you can physically and mentally. It should be hard the whole time, on the verge of blowing up.
I have a CTL of 130 I can start trading in for a bunch of intensity now. More VO2 tomorrow, 8x3.