I'd be surprised to see anything above 3.7 GHz (single core turbo)/3.4 GHz (all-cores active turbo, barring tinkering with base clock 100 MHz upwards on certain mainboards, anyway...(if base clock were adjusted to 104 MHz, one might see about ~3848 MHz under single core tasks.
That being the case, I've not personally seen task manager that far off on showing at least the top speed currently utilized on one core, even if slightly slower to update than some monitoring software..