bob prohaska  wrote:
> Just got a Pi4 with 8GB of RAM up and running. It's an impressive
> upgrade from a 3b+....
>
> It looks as if the logic of the green status LED has been inverted:
> On the 3b+ the LED was off when idle, flashing when active. The 4
> seems to idle with the green LED on, it blinks off when active.
>
> Is that the whole story? I poked around a little on the Web and
> didn't find any mention, except for flash codes that signal POST
> failures.

Look in the file /boot/overlays/README. Well I might as well just
copy the potentially relevent bits here, but I may have missed some:

        act_led_trigger         Choose which activity the LED tracks.
                                Use "heartbeat" for a nice load indicator.
                                (default "mmc")

        act_led_activelow       Set to "on" to invert the sense of the LED
                                (default "off")
                                N.B. For Pi 3B, 3B+, 3A+ and 4B, use the
act-led
                                overlay.

        act_led_gpio            Set which GPIO to use for the activity LED
                                (in case you want to connect it to an external
                                device)
                                (default "16" on a non-Plus board, "47" on a
                                Plus or Pi 2)
                                N.B. For Pi 3B, 3B+, 3A+ and 4B, use the
act-led
                                overlay.

        pwr_led_trigger
        pwr_led_activelow
        pwr_led_gpio
                                As for act_led_*, but using the PWR LED.
                                Not available on Model A/B boards.

        N.B. It is recommended to only enable those interfaces that are needed.
        Leaving all interfaces enabled can lead to unwanted behaviour (i2c_vc
        interfering with Pi Camera, I2S and SPI hogging GPIO pins, etc.)
        Note also that i2c, i2c_arm and i2c_vc are aliases for the physical
        interfaces i2c0 and i2c1. Use of the numeric variants is still possible
        but deprecated because the ARM/VC assignments differ between board
        revisions. The same board-specific mapping applies to i2c_baudrate,
        and the other i2c baudrate parameters.

Name:   act-led
Info:   Pi 3B, 3B+, 3A+ and 4B use a GPIO expander to drive the LEDs which can
        only be accessed from the VPU. There is a special driver for this with
a
        separate DT node, which has the unfortunate consequence of breaking the
        act_led_gpio and act_led_activelow dtparams.
        This overlay changes the GPIO controller back to the standard one and
        restores the dtparams.
Load:   dtoverlay=act-led,=
Params: activelow               Set to "on" to invert the sense of the LED
                                (default "off")

        gpio                    Set which GPIO to use for the activity LED
                                (in case you want to connect it to an external
                                device)
                                REQUIRED



        eth_led0                Set mode of LED0 - amber on Pi3B+ (default
"1"),
                                green on Pi4 (default "0").
                                The legal values are:

                                Pi3B+

                                0=link/activity          1=link1000/activity
                                2=link100/activity       3=link10/activity
                                4=link100/1000/activity  5=link10/1000/activity
                                6=link10/100/activity    14=off    15=on

                                Pi4

                                0=Speed/Activity         1=Speed
                                2=Flash activity         3=FDX
                                4=Off                    5=On
                                6=Alt                    7=Speed/Flash
                                8=Link                   9=Activity

        eth_led1                Set mode of LED1 - green on Pi3B (default "6"),
                                amber on Pi4 (default "8"). See eth_led0 for
                                legal values.

That's from a quite recent RPi OS installation.

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