We've gotten used to the GPIO-available functions of Raspberry Pi computers remaining largely the same over the years, which is why it might have flown a little bit under the radar: the Raspberry Pi 4
Configure Serial Raspberry Pi 4B - UART - GPIO 14 and 15
USB TO UART/I2C/SPI/JTAG - Waveshare Wiki
Raspberry Pi: C++ Libraries for working with I2C, SPI and UART, by Sebastian, Geek Culture
Serial Expansion HAT for Raspberry Pi, I2C Interface, 2-ch UART, 8 GPIOs
reRouter carrier board
Everything You Need to Know About the Raspberry Pi 4
Raspberry Pi Compute Module 4 Review: Small Yet Mighty
SPI to 4 X UART Bridge (MULTIUART) : 6 Steps - Instructables
Raspberry Pi SPI and I2C Tutorial - SparkFun Learn