You can set your server to reboot every 4 hours by setting the following crontab.

First you may want to select your preferred editor otherwise vi maybe a tad too complex for someone just starting. To that first:

sudo update-alternatives --config editor

Then edit your crontab:

sudo crontab -e

There you can set your server to reboot every 4 hours with the following:

# For more information see the manual pages of crontab(5) and cron(8)
# 
# m h  dom mon dow   command  

0 */4 * * * root /sbin/shutdown -r now

However, you should not have a need to reboot your server every 4 hours. If you are running into that then you have a larger issue that needs to be investigated. Perhaps you have a memory leak, or something else on your server is overloaded.

You should be able to run a server for months without any service interruptions. Technically years, however you will need to reboot for a security update.

I edited file for auto reboot Ubuntu 18 server in every 6 hours. Is that ok?

# Edit this file to introduce tasks to be run by cron.
#
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
#
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
#
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
#
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h  dom mon dow   command

0 */6 * * * root /sbin/shutdown -r now

It’s not working for me, on Ubuntu 18.04..