Impossible difficulty forces you to fight longer with first tier creatures thus have to micromanage your casualties as well as the monsters you pick to defeat. No shooters early, so you need melee combat skill.
Yes, H4 offers more melee and soldier defeat. ERA could have make a first strike, simultaneously melee and archery, negative first strike. WoG is half-trip to H4. And you develop others. No Heroes. Later on I install ERA. I've never experienced it. Ok yes harder, it also causes to less amount of monster. If you and team make them. And player chooses enable to do H4 in the battlefield or disable option. So depends on old maps
Fight MWMs - stand teach