I have done A LOT of treadmill hill reps.
4% to me was something where I noticed an uptick in respiration rate and intensity but not enough to make me concerned if I was doing a zn2 type run. 5% and 6% elicited a stronger response. but I personally have something like 4min @4% + 1min @5% + 1min @6% for 30-40 minutes straight and never had it feel like a threshold workout, maybe a tempo.
I think some of this is going to come down to how long the workout is, as you know the greater the duration they are running uphill the greater the chance of them having a workout feel like tempo or threshold. The runner themselves: how good they are at pacing ie do they tend to run at the top of zn2 all the time or are they more willing to just go run easy on those bread n butter type runs, how good they are at/on hills (oddly I suck at them in a race), are they manipulating the speed or not and if doing repeats the recovery.
Hope that helps give you a better framework. email me (it's in my profile) if you have more specific questions as I write a ton of treadmill workouts for my athletes, especially this time of year and have had athletes that did 95% of their yearly run training on a treadmill
Brian Stover USAT LII
Accelerate3 Coaching Insta