For 4 yrs old, I would vote Magic -- more "fun".
We had our son in Milton Magic for a couple of years, then moved him over to the Academy when he was 6 since he wanted something more competitive -- if your kid goes a little beyond the "fun fun hooray everybody's a winner" stage, the Academy becomes the better fit. The Academy seems more geared towards skills development, and definitely a path to more serious soccer.
I agree with Andrew, week in and week out, you saw some coaches who were fantastic, and others that didn't seem to want to be there unfortunately. That was the only downside.
My kid loved it, and the Academy asked us to put him in the more advanced ADP team or whatever they called it - but at 3 nights a week minimum, that would make him a soccer kid and nothing more, and that's where the rougher play and more hard core sports-parenting is found
... and we're not getting into anything that serious yet. He is 7 this year, and we are going back to the Academy (but the regular house league version), and I think it will be a really good fit. Still challenging but still fun (I hope!).At 4, I'd vote for the Magic - our guy had fun there - after that age, we preferred the Academy.
Good luck either way!