Hi sir,
The Base clock for the 3600 is 3.6Ghz with a boost clock of 4.2Ghz on ONE or TWO cores for short bursts depending on your temperatures. I assume you see 4125-4200Mhz on one or more core at one time while other cores rest comfortably near 3600-3800Mhz. This is normal. Ryzen tends to boost only a single core out of the six you have to the rated boost speed that too only and only if the temperatures are in check.
Coming to your other question about overclocking, the chip is relatively new meaning less guidance for overclocking. Getting a stable all core overclock is hard. Managing a 4Ghz overclock on all six cores would be a great overclock but will require high voltage and massively increase the temperature resulting in instability.
