You can use the 0.58x BMPCC Speed Booster on the BMPCC 4K. It mentions N/A**, can be mounted, but does not cover the full sensor, on the Metabones website's grid.
https://www.metabones.com/a/compatibilityGenerally, it won't vignette on the 2.6K S16 and HD sensor area options. Old reviews of the camera may not mention these options. This includes the Metabones grid on their website, since they only list 1.43x and 1.29x, not the 1.75x crop factors. This is because the 2.6K sensor option was introduced in a late 2019 firmware update, and not available in the 2018 introduction of the BMPCC 4K.
A used Metabones BMPCC 0.58x Speed Booster can be a real bargain compared to an XL or an Ultra, if you don't mind avoiding the full 4K sensor. It may be sharper than a Viltrox, or other MFT focal reducer at infinity, since it is not designed for an OLPF filter, and mounts closer to the sensor.