I created a cron job as root with the command:

crontab -e

Then I added 0 4 24-31 * 4 /home/backupscript.sh after the last line.

The output of crontab -e looks like this:

# 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 4 24-31 * 4 /home/backupscript.sh

My script at /home/backupscript.sh looks like this:

cd "/home/backups/"

datestring="$(date +%Y-%m-%d)"

sudo /opt/bitnami/ctlscript.sh stop

sudo tar -pczvf ${datestring}.openproject-backup.tar.gz /opt/bitnami

sudo /opt/bitnami/ctlscript.sh start

It works fine when I run it with bash /home/backupscript.sh.