The CPi-C series panel PCs have a 26-pin header socket on the rear panel with the following pinout.

See pp.8 of the Compute Module 4 datasheet for more information.

The GPIO pins are not exposed through a level shifter like the CPi-S panel PCs, so the internal pull-up/-down resistors can be utilized. GPIO45 has an internal 1.8kohm pullup resistor to 3.3V because it is initially used for I2C (See Table 6 of the Compute Module 4 datasheet).

The 5V pins can supply a maximum current of 1A. The 3.3V pins can supply a maximum current of 700mA. If both 5V and 3.3V rail pins are used simultaneously, their total power should be kept below 5 watts.

The panel PC's RTC is connected to I2C1, so those pins assigned to I2C1 should not be reconfigured.