The Raspberry Pi 4 stack of HATs, Bonnets and custom boards starts with the Raspberry Pi 4 and Rainbow Pibow Coupé 4 case.

Raspberry Pi 4 and case

The fan and custom board sets on top of the case, against the 2x20 header and held in place by the screw in the lower left corner. The lose connector is plugged to the 16 PWM/Servo HAT that goes on next.

Raspberry Pi 4 with fan added

The 16-Channel PWM/Servo HAT is mounted above the fan board. In the middle of the HAT are 6 addressing pads used to distinguish this board from other boards using the I2C Raspberry interface. None of the pads are shorted here therefore, address 0.

16 3-pin connectors across the front of the PWM/Servo HAT are the outputs to control 16 servos. The control and 5vdc on the first connector are used to control the fan. 

On the right edge of the (blue) board are connectors for 1) 4 traffic monitor LEDs, 2) 4 tri-color LED signals and 3) push-button to control the fan. For details see 16-Channel PWM/Servo HAT.

On the left edge are two black connectors for 5vdc to drive the servos connected to this board separate from the 5vdc used by the Raspberry Pi and on the 2x20 pin header. The plug is used to input 5vdc and the terminal block is used to daisy-chain the voltage to the PWM/Servo Bonnets that are next in the stack.

Raspberry Pi 4 with Servo/PWM HAT on top

The first of three 16-Channel PWM / Servo Bonnets is connected to the raspberry Pi 2x20 header next. The Bonnets have the same functionality as the PWM/Servo HAT, capable of controlling 16 servos. The bonnets do not have the modification area on the right side as does the HAT. Notice the 6 addressing pads middle right of the bonnet. In this case the low order bit, A0, is shorted for address 1 on the I2C bus. 5vdc is linked up from the PWM/Servo HAT by the red/black wires on the left. 

Raspberry Pi 4 stack with first Servo Bonnet added

Next in the stack are the second and third Servo Bonnets. They are addressed 2 and 3 using the 6 addressing pads.

Raspberry Pi 4 stack with second Servo Bonnet addedRaspberry Pi 4 stack with third Servo Bonnet added

Finely an interface board is on top of the stack to connect to the two Unicorn displays. The 11-pin header brings out 3v3dc(1), 5vdc(2), grounds(3) and SPI interface(5) pins. 

Full Raspberry Pi 4 stack with custom board on top