In both Debian 9 (stretch) and Debian 10 (buster), the process is started by the following two systemd timers:

• apt-daily.timer, which via apt-daily.service calls /usr/lib/apt/apt.systemd.daily update to update the package lists (apt-get update), and
• apt-daily-upgrade.timer, which via apt-daily-upgrade.service calls /usr/lib/apt/apt.systemd.daily install to install the upgrades (unattended-upgrade).

(The anacron job /etc/cron.daily/apt-compat still exists, but exits if it detects systemd. Without systemd, it will run /usr/lib/apt/apt.systemd.daily without sub-command, which means both update and install. See other answers or anacron documentation on changing the schedule if you don't use systemd.)

The systemd timers can be set to trigger at a higher frequency, in your example every four hours, by overriding the default schedule. First, for updating the package list:

$ systemctl edit apt-daily.timer
This creates /etc/systemd/system/apt-daily.timer.d/override.conf. Fill it as follows:
 
[Timer]
OnCalendar=*-*-* 0,4,8,12,16,20:00
RandomizedDelaySec=15m

Then, for the actual upgrades 20 minutes later:

$ systemctl edit apt-daily-upgrade.timer
 
[Timer]
OnCalendar=*-*-* 0,4,8,12,16,20:20
RandomizedDelaySec=1m

To check your work:

$ systemctl cat apt-daily{,-upgrade}.timer
$ systemctl --all list-timers apt-daily{,-upgrade}.timer

(Taken partly from Debian Wiki: UnattendedUpgrades.)