The spirit of Heroes is a special substance; there are many indicators, but the main thing is that the hero controls the army, and he himself only uses magic. Also, the spirit of Heroes is consistent throughout the game. The first three parts are very similar, with small improvements and additions, but the fourth part is already a foreign game in this chain. In the fifth part, they had the sense to bring the game series back on track. In the sixth part, the spirit of Heroes was somewhat spoiled by reducing resources, but this is not as critical as the atrocities in the fourth part!
I have expressed my opinion, and that's it; you can love Heroes 4 and play it, it's your right.
And regarding playing as a hero without an army. What prevents you from taking one hero with an army and playing that way? For example, I prefer to play with an army, not with individual heroes. And the game mechanism is not too different from the third game, if only you understand it.
Справочник по НоММ-4 http://handbookhmm.ru/pamyatka-dlya-heroes-iv.html
Памятка по НоММ-3 http://handbookhmm.ru/pamyatka-dlya-homm-iii.html
и HotA http://handbookhmm.ru/vyishla-obnovlennaya-pamyatka-dlya-homm-iii-hota.html