I was so confused how you were managing to do 4:00 pace yesterday in zone 1. That's some Collings level of fitness! Glad to see you're mortal after all.
It's actually a bit weird with the optical it's a step function around the 2km mark, 2km in zone 1-2 then it jumps up and puts the last km in zone 5. HR strap gives a linear, steadily increasing line, as you'd expect I suppose.