In the last episode of season 3, we don't see El and Mike breaking up (because El done that in the second episode) but rather we see her telling him, "I love you too." But that was before she read the Heart-to-Heart Speech by Hopper and bursting into tears (+ the fact that she is moving out with the Byers and leaving her friends behind in Hawkins).
In Season 4, if the Mind Flayer (or the Demogorgon) returns and the kids go through Hell in Hawkins for another time then El and Mike may have second thoughts about their relationships. El, doesn't want to continue her relationship with Mike fearing that he will put himself in danger for her and hurt or kill himself in the process. And Mike because she has already went through an emotional rollercoaster (not a real one though) last season because of losing Hopper and he cannot for the second time this happens. As a result, they may try to put a halt in their relationship. But have no fear, as our favorite PI/Conspiracy Theorist/Part-Time Cupid comes to their rescue. He will probably do what he did with #Jancy and #Jopper to #Mileven and fix it which will lead it from them breaking upo to making up.
What do you think?