In an attempt to help Mike Wheeler understand his importance to Eleven, Will’s monologue includes a reference that Mike is the heart of their friend group. Unfortunately, although that may have been true in season one, or at least closer to it, season 4 does no favors in backing up Will’s statement. It actively seems to go against that claim.
Mike is essential to Will, and it could be suggested that Will is blinded by his perspective of Mike as he makes his statement. Mike is also vital to Eleven as he was the first person to treat her with genuine kindness and no cruel intentions. But, Mike does not seem to be the heart of the group where season 4 is concerned.
Mike spends most of the fourth season isolated from the main story and most of his friends. Instead of Mike, the group seems to rally around Dustin Henderson as the heart of the group.
It is Dustin’s friendships with everyone that brings the entire group together. While he spends some time in Hawkins, Mike does not have the same effect on the characters that Dustin seems to. Granted, Will has not been in Hawkins to witness any of this behavior either.
Eleven and Will rely on Mike as a close companion to both of them, potentially beyond anyone else. However, to the rest of the group, Mike does not seem to be a frontrunner, at least not in season 4. No one seems all that concerned with what Mike is up to, and the group functions just fine without him at the center of everything.
Given the events of season 4, Mike is not even at the heart of the show. Instead, that role could go to Max Mayfield rising as the season’s most valuable character, Eleven’s eventual return to her powers, and fan-favorite Eddie Munson, all of whom have far more exciting things occurring for them than Mike Wheeler.