I'm somewhat confused by the documentation on strapping pins. I think there's a mistake.
Rev 1.5 of ESP32 Datasheet section 2.4 discusses them. It says there are 5 while Rev 2.2 of the Technical Reference Manual shows 6 bits used in the GPIO_STRAPPING register. The ESP32 Datasheet also has MTDO doing 2 things, controlling Debugging Log on U0TXD and setting the Timing of SDIO Slave. I think GPIO4 should probably be one of those to match the TRM, but it's not clear which one.