Hm, I was not being particularly precise with my language because I was trying to make my explanation easily digestible, but please correct me if I'm wrong.
The null hypothesis is that there are no new particles or physics and the Standard Model predicts the magnetic charge of a muon. A 4.2 sigma result means that given this null hypothesis prediction, the chances that we would have observed the given data is ~0.0013% (chance this was a statistical anomaly). Since this is a vanishingly small chance (assuming no experimental errors), we can reasonably reject the hypothesis that the Standard Model wholly predicts the charge of a muon.
The null hypothesis is that there are no new particles or physics and the Standard Model predicts the magnetic charge of a muon. A 4.2 sigma result means that given this null hypothesis prediction, the chances that we would have observed the given data is ~0.0013% (chance this was a statistical anomaly). Since this is a vanishingly small chance (assuming no experimental errors), we can reasonably reject the hypothesis that the Standard Model wholly predicts the charge of a muon.