In Thirds, is the hydra a sausage like in IV?
In Thirds, is the minotaur a naked bull like in IV?
In Thirds, are the nagas Japanese with blades?
Compare the heroes: take Ajit (Chaos), for example. In Thirds he was a lord, but in Fourths—a thief?! Ivor is a monk, there he's a priest (lower status). How can anyone even perceive same Fourths as a continuation of the series.
BUT: If Fourths had been released under a different name, it would have had more success, IMHO.