For your reference:
sudo nano /etc/systemd/system/systemd-reboot.timer
Paste all within the lines into the file:
[Unit]
Description=Reboot daily at 4:00
[Timer]
OnCalendar=*-*-* 4:00:00
[Install]
WantedBy=timers.target
STRG+O = Create file
STRG+X = return
cat /etc/systemd/system/systemd-reboot.timer // Check
sudo systemctl enable systemd-reboot.timer // start timer automatically after a reboot.
sudo systemctl start systemd-reboot.timer // start timer immediately
Info:
sudo systemctl stop systemd-reboot.timer // stop timer immediately
sudo systemctl restart systemd-reboot.timer // restart timer if it is already running
sudo systemctl status systemd-reboot.timer // shows status of the timer