My personal prediction is that the MC won't hit Level 4 until he encounters Indra. A monster that wields his greatest weakness and killed his mother is the perfect trigger for the "physical and/or emotional trauma" condition for evolution. And if the MC getting to Level 4 finally lets him get control of his response to electricity, it would make him a natural counter to Indra and potentially make it possible to beat him at just Level 4. Taking out one of the Chosen would definitely count as one of the victories against strong opponents that Ella is hinting he needs to reach Level 5.
I expect that this upcoming arc in the monster world will serve mostly to get the rest of the party up to Level 3. The MC will still get stronger, he's got new applications of his memory powers to explore and being around so many monsters might see him finally start making monster anatomy weapons. But the big moments of growth will come from Michael, Alice, and Amber/Liz. Michael's been established as having a weird response to the energy of the monster world and could wind up evolving from that, Alice is likely going to have to push herself to her limits to activate her ring and get the team home, and if Amber and Liz evolve even once they'll hit Level 3 immediately from the twin resonance. So the team will likely come out of the monster world as a group of strong Level 3s, the whole party on the MC's level and able to fight alongside him effectively. We saw that two well-coordinated Level 3s could take out a Level 4. Let's see what six of them can do.