The Marvel epic will serve as the follow-up to Avengers: Infinity War, which has made more than $2 billion worldwide and is one of the most successful films of all time.
Markus and McFeely joked that riding that wave has made going back to work even harder.
“There’s time for reshoots and fixing and we’re scrubbing through it.
“The Russo brothers [directors] have been in there [the editing room] for a few weeks now and the team’s getting together tomorrow.”
They added: “It’s disheartening to get off the one that’s all done, and get in a room and it’s just full of parts.
“You know? It’s like, ‘ah, s**t. It’s not done.’”
They had earlier spoken in the same podcast episode about reading theories surrounding the whereabouts of the Soul Stone – one of the big talking points amongst fans prior to the release of Infinity War.
“We wanted it somewhere meaningful to Thanos – pure to the movie so that it wasn’t, ‘and now we’re going over here!’” they explained.
“It was really important to have the mystery – what does Gamora know that you all don’t know?
“It [theories] doesn’t affect what we write. Boy, you just can’t do fan service that way.”
Avengers 4 will pick up from Infinity War’s jaw-dropping cliffhanger, which saw multiple MCU favourites being killed off when Thanos collected all of the Infinity Stones.
The sequel will add the likes of Jeremy Renner as Hawkeye, Paul Rudd as Ant-Man, Evangeline Lilly as the Wasp and Brie Larson as Captain Marvel.
Avengers: Infinity War is still playing in cinemas now.