The carrier board for Raspberry Pi Compute Module 4 with integrated ATMEGA328PB MCU, RTC and EEPROM. The design includes power source and basic connectivity - Ethernet, HDMI, USB2.0. I had to use 4 layer board because of the HDMI, USB and Ethernet differential pair impedance requirements. And even though I did not pay for the Impedance Control option all the pairs seems to be working really fine. The board also contains the fine pitch connector for CM4 (0.4 mm pitch) and it is working like a charm... I was also pushing the selected limits (Min Track/Spacing : 6/6mil, Min Hole Size : 0.3mm) and I have not found any problems so far.... The 4 layer PCBs are more expensive than from some competitors but they are really good made....