As stated in the comments, your processor support Intel Turbo Boost Technology.
If you are not scared by (very slightly) technical papers, I suggest you to take a quick look at that paper.
Your CPU is the 3770 which is designed to operate safely1 at 3.40 GHz.
However such CPU has four cores, as less cores are active the total thermal power lowers; so the Intel engineers though well that they can safely increase the clock of the active cores.
Intel Turbo Boost basically increases the clock of the CPU up to 3.90 GHz as long as the power/current/thermal consumption stays in the safe range.
Paradoxically this is possible only when the CPU is somewhat idle, as explicitly stated in the paper linked before:
For example, one particular processor may allow up to two frequency steps (266.66 MHz) when just one core is active and one frequency step (133.33 MHz) when two or more cores are active. Therefore, higher deep C-state residency (“C3” or “C6”) on some cores will generally result in increased core frequency on the active cores.
Don't make the mistake to think that a higher speed means a busier CPU. It's more the opposite!
As a rule of thumb, the CPU can use a higher clock as long this is for short periods or with non very demanding2 software.
The real thing is a closed-loop control
These constraints are managed as a simple closed-loop control system.
When temperature, power or current exceed factory configured limits and you are above the base operating frequency, the processor automatically steps down core frequency in order to reduce temperature, power and current. The processor then monitors temperature, power, and current and continuously re-evaluates.
Intel Turbo Boost Technology is active as long as the Power profile is what is technically called P0 (in ACPI jargon).
This means that if you enable "Max performance" in the settings of the OS, than the processor stays at 3.90 GHz on idle.
Regarding the measure of 4.14 GHz from task manager, I am very skeptical about overclock.
Intel processors don't have a direct way to report the current operating frequency. In the same old paper linked at the beginning, Intel gives an algorithm to estimate the running frequency and says:
Due to the way the BIOS and OS communicate Intel® Turbo Boost technology, software may never detect core clock frequencies above the base operating frequency
Being an esteem it can be off by some value, 4.14 GHz over 3.90 GHz is a 6% error which to me is not enough to indicate an overclocking, though I may be wrong here.
I'm not even sure the 3370 as an unlocked multiplier (should end in k?)
My guess is that Task Manager is doing is best to estimate the current frequency but it is now always accurate.
1 In terms of current/power consumption and temperature.
2 In the sense that not all functional units of the processor are used.