quick&dirty but generic solution to scale external monitors connected to a (4k) laptop

90-plugdisplay.rules

# /etc/udev/rules.d/90-plugdisplay.rules
# may have to be modified if you e.g. have more than 1 graphics card
KERNEL=="card0", SUBSYSTEM=="drm", ACTION=="change", RUN+="/usr/local/bin/plugdisplay.sh"

plugdisplay.sh

#!/bin/bash
# system-wide /usr/local/bin/plugdisplay.sh
# to be triggered by udev rule for display plug event

sesinfo() {
	loginctl show-session ${1} | grep 'Name\|Display' | sed -e 's/\([A-z]*\)/\U\1/'
}

for s in $(loginctl | awk '$4 ~ /seat.*/ { print $1 }'); do
	unset DISPLAY
	eval $(sesinfo ${s})
	if [ -n "${DISPLAY}" ]; then
		echo "Running display plug event for user ${NAME} on display ${DISPLAY}"
		# wait some time for monitor to be recognised by DE
		(sleep 8 && su - ${NAME} -c "DISPLAY=${DISPLAY} "'${HOME}/.local/bin/xrandr-scale.sh') &
	else
		echo "NAME==${NAME} DISPLAY==${DISPLAY}"
	fi
done

xrandr-scale.sh

#!/usr/bin/env bash

# set -x

getdisphres() {
    xrandr --current | grep '*' |  awk -v line="${1}" 'NR==line{print $1}' | cut -d 'x' -f2
}

getnumdisp() {
    xrandr --current | grep -B1 -A0 '*' | grep connected | wc -l
}

getnthdispname() {
    xrandr --current | grep -B1 -A0 '*' | grep connected | cut -d ' ' -f1 | sed "${1}q;d"
}

y1=$(getdisphres 1)

for d in $(seq 2 $(getnumdisp)); do
    y=$(getdisphres ${d})
    n=$(getnthdispname ${d})
    f=$(bc -l <<< "scale=2; ${y1} / ${y} / 1.5 * 2.0")
    xrandr --output ${n} --scale ${f}x${f}
done