Ash met different E4 members during different times. So the Flint that Ash met at that time was significantly weaker than the Malva we are seeing now?What are you saying dude? I said that all the E4 are equal, because they started on an equal level, thus were always on the same level. If you're very good, then there is less room to improve, so even if all the E4s improved over the course of Ash's journey, it was a marginal improvement
If their improvement is significant, that would contradict the anime since Flint of the E4 was powerful enough 2 sagas ago to one-shot Ash's infernape.
If their improvement is marginal, then Siebold would have barely improved since facing MCX, and would likely have been on Malva's level in ME act IV. MCX however has drastically improved, as it beat Malva after facing 9 ME trainers, and would easily beat Siebold as well.
No matter which theory you go with, it will still end up with MCX > E4