adding these lines to the /boot/config.txt works like a charm for me on a RPi 4 (PI OS 2020-08-20)

#disable ACT and PWR LEDs
dtparam=act_led_trigger=none
dtparam=pwr_led_trigger=none

#disable ethernet port LEDs
dtparam=eth0_led=4
dtparam=eth1_led=4

Cheers