Yeah, a CPU's clock rates will increase or decrease depending on whether the processor is under load or not, with the clocks dropping under light workloads to avoid wasting power and generating excess heat when additional performance isn't needed. Additionally, the clock rates under load will vary depending on how many cores are active, along with other factors like the temperature of the processor.
If you run some demanding task on the CPU, you are likely see the clock rates increase closer to 4GHz.