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Same size, different layout
As you can see, while the Raspberry Pi 4 Model B, on the left, is the same size as the Raspberry Pi 3 Model B+, it has a different layout.
The change has been necessary to incorporate the dual micro-HMDI ports on the board.
The new layout means side panels in cases for older Pi boards won’t fit correctly. I also found the board overheated when playing video while using an older case, with the computer throttling performance to reduce temperature.
On the back
The underside of the Pi 4 is more bare than previous boards, housing only the microSD card reader.
Wi-Fi and Bluetooth
Wireless connectivity is similar to the Pi 3 B+, with the Raspberry Pi 4 offering dual-band 802.11ac Wi-Fi, but Bluetooth is upgraded to 5.0, compared to the Bluetooth 4.2 found in the Pi 3 B+.
The board can be used with two monitors at once via its dual micro-HDMI ports. These are able to support up to twin 4K@30Hz screens or a single 4K@60Hz monitor.
As with previous boards, the Pi 4 also includes a 4 pole stereo audio output and composite video jack.
USB 3.0 arrives
The Pi gets faster ports for hooking up storage and other peripherals, with 2 x USB 3.0 and 2 x USB 2.0 ports.
The position of the USB and Ethernet ports has changed, with the USB ports now to the left of the Ethernet.
Better hardware support
Like the Pi 3 B+, the Pi 4 also sports a 40-pin, general purpose input-output (GPIO) header for hooking up other boards, sensors, motors, and homemade hardware to the Pi.
The pin layout is identical to earlier versions of the Raspberry Pi and the Pi 4 should be compatible with many hardware add-ons for earlier boards.
The Pi 4’s GPIO header also supports more connections, with UART, SPI and I2C interfaces each supported on four additional pins, and with fixed support for clock stretching over I2C interfaces.
True Gigabit Ethernet
Like the Pi 3 B+, the Pi 4 has a Gigabit Ethernet port, however, unlike the previous board, the Pi 4 offers true Gigabit Ethernet speeds due to not being constrained by a shared USB 2.0 bus.
More display options
Useful for hooking up small LCD screens to the Pi, the usual DSI connector found on Pi boards is also present.
The Pi 4 is the first board to use USB-C for its power, and requires a 5V/3A power supply, giving the Pi 4 an extra half amp power compared to earlier boards.