Did Kishimoto kill Itachi because he was too strong to fight in the Fourth Great Ninja War? I have seen many sites stating Kishimoto has said that if Itachi was alive during the Fourth Great Ninja War he would've easily defeated Madara and that is the reason why Kishimoto had him killed. 

Can anyone point me to some sources for these comments?
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.