z4 is where you cross the threshold in both power zones and the HR zones I use. Should be able to hold z4 for quite some time. Maybe your zones are set too high and you're really up in z5? HR zones are ideally set by finding your lactate threshold. Max is only useful in determining LT, and people vary in where LT is in relation to MHR. It turns out my LT is low compared to my MHR. At least it was when I got a metabolic test a year ago. I estimated my MHR using my LT, and then went way over that max when I started going crit's. I'm going to schedule another test to nail down my aerobic zones for base training.
For instance, in the 5-zone system that I use (based on % of max heart rate), I am lucky if I can hold Z4 for 2-3 minutes at a time and maybe 6 reps. Z5 is an all out sprint for 15 seconds. TIA.