In a blog post from a few years ago, [Peter Molnar] explains in detail how to wire up a physical adapter to add a USB-connected I2C interface to a system. At its core is the ATtiny85 AVR-based MCU ...

the Raspberry Pi 4 has six SPI controllers, six I2C controllers, and six UARTs – all on its 40-pin header. You can’t make use of all of these at once, but with up to four different connections ...