The third game is more well-thought-out as a game; it's easier to predict its further development. The fourth game has its own quirks, for example, the number of enemies increases 4.5 times depending on the difficulty level, compared to 2 times in the third game, which sometimes slows down the start significantly. But this is more of an imbalance than a flaw. Imagine a real situation – cavalry attacks infantry, and they form a tight formation and raise spears – some of the cavalry will cut down the infantry, and some will be impaled on the spears – that's simultaneous attack! You need to change tactics – in the third game, an archer had no protection from other archers, but here, if you cover them with infantry, the infantry will take simultaneous fire from both archers and mages.
I successfully completed the Dangerous Thicket using a Tactician and a Priest. Take a look at the small maps here: http://www.heroesleague.ru/homm4/maps/small/, here: http://www.klubcentres.ru/heroes4/maps.html, and here: http://heroes.ag.ru/maps/.