The Fantastic Four showed up in the Secret Wars three parter during the fifth season. They along with Iron Man, Storm (the original voice of Storm, Iona Morris provided her voice instead of Alison Sealy-Smith), and Captain America come to Spidey's aid. They managed to get Robert Hays (the voice of Iron Man) and James Avery (the voice of War Machine from the first season of the "Iron Man" program) to reprise their parts. They also managed to get the entire cast of "X-Men" to cross-over on "Spider-Man" for two episodes.
However, when it comes to the Fantastic Four, they were only able to get Quinton Flynn, the voice of Johnny Storm (the Human Torch) from Season 2 of the "Fantastic Four" series. For whatever the reasons, Chuck McCann (the Thing), Beau Weaver (Mr. Fantastic), and Lori Alan (the Invisible Woman) are nowhere to be found. Plus, the team is wearing their "Heroes Reborn" uniforms, instead of the uniforms that they wore (even though, I'm aware that the "Spider-Man" series and the "Fantastic Four" series used different animation houses) their own TV series.
However, when it comes to the Fantastic Four, they were only able to get Quinton Flynn, the voice of Johnny Storm (the Human Torch) from Season 2 of the "Fantastic Four" series. For whatever the reasons, Chuck McCann (the Thing), Beau Weaver (Mr. Fantastic), and Lori Alan (the Invisible Woman) are nowhere to be found. Plus, the team is wearing their "Heroes Reborn" uniforms, instead of the uniforms that they wore (even though, I'm aware that the "Spider-Man" series and the "Fantastic Four" series used different animation houses) their own TV series.