If a new manager comes in they'd need some time to adapt a few seasons perhaps, but once they did (providing it was a good manager), I'm pretty confident they'll outperform Wenger. He's not doing a good job, the basic errors he makes are shocking to be honest, 4th place or not his achievements since 2006 are average at best.
You can't expect a new manager to come in and change things just like that, this club is all about Wenger and has been for a long time, it's an unhealthy situation.