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The BCM2711 used in the Raspberry Pi 4 Model B, the Raspberry Pi 400, and the Raspberry Pi Compute Module 4 has 4 additional UART (uart2-uart5) in addition to uart0/1 on the older Pi (only one of which can be used as they share GPIO).
Functionally these are equivalent to the fully featured PL011 UART on uart0 and can optionally be configured with CTS/RTS. See UART configuration for Foundation documentation.

If additional UARTs are enabled they will appear as /dev/ttyAMAn
The first will be /dev/ttyAMA1 and increment if additional UART are enabled.

The BCM2711 used in the Raspberry Pi 4 Model B, the Raspberry Pi 400, and the Raspberry Pi Compute Module 4 has 4 additional UART (uart2-uart5) in addition to uart0/1 on the older Pi (only one of which can be used as they share GPIO).
Functionally these are equivalent to the fully featured PL011 UART on uart0 and can optionally be configured with CTS/RTS. 
See UART configuration for Raspberry Pi Ltd. documentation.

If additional UARTs are enabled they will appear as /dev/ttyAMAn.
In current kernel /dev/ttyAMAn corresponds to UARTn.
In older OS/kernel they were numbered sequentially starting with /dev/ttyAMA1 and incrementing if additional UART are enabled.

NOTE you can enable overlays on the fly e.g sudo dtoverlay uart4.
Manually loaded overlays can be removed e.g sudo dtoverlay -r uart4
Changes to the OS mean that enabling some overlays on the fly is not currently working, although formerly it worked.

NOTE you can enable overlays on the fly e.g sudo dtoverlay uart4.
Manually loaded overlays can be removed e.g sudo dtoverlay -r uart4
Changes to the OS mean that enabling some overlays on the fly is not currently working, although formerly it worked.

NOTE you can enable overlays on the fly e.g sudo dtoverlay uart4.
Manually loaded overlays can be removed e.g sudo dtoverlay -r uart4

NOTE you can enable overlays on the fly e.g sudo dtoverlay uart4.
Manually loaded overlays can be removed e.g sudo dtoverlay -r uart4
Changes to the OS mean that enabling some overlays on the fly is not currently working, although formerly it worked.