There is a setting in /boot/config.txt called over_voltage but the Zynthian image only sets this for Pi 2.
I have found that differnt revisions of the Pi 4 behave differently (and sometime unexpectedly) with this setting. Adding the setting under a [pi4] section adjusts the core voltage on a Pi 4 Rev1.2 BCM2711 only if the setting is over 0. Any other setting results in a core voltage of 0.8375V. A Pi 4 Rev 1.4 BCM2835 will adjust its core voltage in sympathy with this setting as expected.
I believe we can optimise the Raspberry Pi 4 to give better performance by adjusting its core voltage and clock speed.
Be aware that setting force_turbo and over_voltage > 0 will set a flag within the SoC and invalidate the warrantee of the Raspberry Pi. @jofemodo, I am not sure if you intend to change the config.txt settings but if you do set values that invalidate Raspberry Pi warrantee then I recommend a prominent warning.
force_turbo is enabled in the Zynthian image. This forces the clock speed whether CPU is busy or not, i.e. disables kernel frequency throttling.
I investigated this because I have been experiencing worse xrun performance on one of my RPi 4 devices. It transpires that I have a Rev 1.2 and Rev 1.4 with the Rev 1.2 performing worse. (Tip: Do not perform rpi-update as this will move you to the 5.x kernel which will break the Zynthian, specifically jackd. I am now reflashing the Zynthian image 
. )
Unfortunately setting over_voltage=1 for the Rev 1.2 board (which sets core voltage to 0.8625V) has not resolved my xrun issues. I will play with these settings to see if I can squeeze the required performance out of this unit.
