Did Kishimoto kill Itachi because he was too strong to fight in the Fourth Great Ninja War?
I don't think that's true. While Itachi is powerful, he is no match for Madara Uchiha, who defeated the 5 kages himself. While Itachi's Susanoo contains the Totsuka blade and a nearly invincible shield, Madara has the perfect Susanoo, which makes it a league of its own.

Even so, Itachi did appear in the Fourth Shinobi War, and while he made Kabuto un-reanimate all the Edo Tenseis (including Itachi himself), Edo Tensei Madara was not stopped.