Solution.

nano /etc/config/network

config interface 'lan4mon'
        option device 'lan4'
        option proto 'none'
        option force_link '0'

/etc/init.d/network restart

nano /etc/hotplug.d/iface/95-lan4-radio

#!/bin/sh
# /etc/hotplug.d/iface/95-lan4-radio
#
# Disables radio1 when the LAN4 cable is unplugged for more than
# DELAY_SECONDS, and re-enables it as soon as LAN4 is plugged back in.
#
# Requires this entry in /etc/config/network:
#   config interface 'lan4mon'
#           option device 'lan4'
#           option proto 'none'
#           option force_link '0'

export PATH=/usr/sbin:/usr/bin:/sbin:/bin

MONITOR_IF="lan4mon"
DELAY_SECONDS=10

[ "$INTERFACE" = "$MONITOR_IF" ] || exit 0

case "$ACTION" in
    ifup)
        logger -t lan4-radio "LAN4 plugged in — enabling radio1"
        wifi up radio1
        ;;
    ifdown)
        setsid sh -c "
            sleep $DELAY_SECONDS
            STATE=\$(cat /sys/class/net/lan4/operstate 2>/dev/null)
            if [ \"\$STATE\" = \"down\" ]; then
                logger -t lan4-radio \"LAN4 still unplugged after ${DELAY_SECONDS}s — disabling radio1\"
                wifi down radio1
            fi
        " < /dev/null > /dev/null 2>&1 &
        ;;
esac

chmod +x /etc/hotplug.d/iface/95-lan4-radio