The Raspberry Pi 4 stock speed is 4 ARM Cores at 1.5GHz. The current max safe overclock is to 2.147GHz. The onboard GPU can be overclocked from 500MHz to 750Mhz safely with proper cooling.
Your results may vary, but for most people, you should be able to use these settings without a problem. I highly recommend a case with a fan if you are running overclocked for any sustained load.
As I am going to use a fan in this configuration, I'm going to use this profile as our baseline.
This is a Raspberry Pi 4 at stock clocks and in a case with a 5V fan. As you can see from the chart we maintain 1.5GHz for the entire test and the temperatures never get over 54C. This is a fantastic profile for a 100% load over 30 minutes.
I recommend reading my Raspberry Pi 4 cooling post, but if you want the cliff notes, you can undervolt the fan and reduce 98% of the noise while maintaining zero throttling under full stress.
The trick here is to plug the positive (red) cable of the fan to the 3.3V pin instead of the 5V. There is very little loss in cooling but a massive reduction in the fan whine (which is noticeable at 5V). Each fan is different and using a Noctua fan will result in even better results at a higher price. You can actually do the same trick with a 12V fan undervolted to 5V but you will have trouble finding 12V fans that can fit in a Raspberry Pi case.
Testing 2.147GHz overclocks
I am going to test the maximum overclock with an undervolted fan and then a full 5V to the fan and see how well they can handle the overclock without throttling.
Test 1 - 2.147GHZ CPU 750GHz GPU 3.3V Fan
This first test is using a full maximum overclock using a 3.3V fan trick. Under these conditions, I was seeing a lot of throttling all the way down to 750MHz. While this may perform perfectly fine for 98% of use cases, I want to test the worst-case scenario and go from there.
Test 2 - 2.147GHz CPU 750GHz GPU 5V Fan
When running a 40mm fan at 5V it is able to keep it below the throttling threshold, but just barely. Between 80C-85C the Raspberry Pi 4 will start to throttle. In this case, the temps hovered around 79-81C but not high enough to trip the throttling. The entire 30-minute test was at a maximum clock of 2.147GHz.
If you are not running GUI applications, I would recommend keeping the GPU clocked at 500Mhz which will further reduce the temps.
While I would 100% recommend a fan if you are overclocking, you likely will still be fine undervolting the fan to 3.3V as long as you are not pegging the CPU to 100% for more than 5 minutes at a time.
How to Overclock the Pi 4
This is the easy part, just edit
Down in the [Pi4] section add these three lines.
over_voltage=6 arm_freq=2147 gpu_freq=750
Be sure you do not have any of these settings already in your configuration file.
Reboot the Pi.
If you have any problems, adjust the frequency. Going over 6 with over_voltage will void your Pi warranty.
How to really cool your Raspberry Pi 4
If you want to use the maximum overclock and you want to run a full 100% load for sustained times, you can go extreme and use the Ice Tower cooler for around $20 on Amazon.
While it does look ridiculous, it does do a great job at cooling and looks really cool! (ok, I'll see myself out.)