Hope following will work
First let’s do a full system backup
dietpi-backup
once done we can start migration to Sonarr v4
cd /tmp/
wget https://raw.githubusercontent.com/Sonarr/Sonarr/develop/distribution/debian/install.sh
chmod +x install.sh
on the installer, we need to adjust 2 line. You can use nano or any other text editor
around line 35 set
app_prereq="curl sqlite3 wget libicu72"
around line 43 set
datadir="/mnt/dietpi_userdata/$app/"
save and exit the file. Ready to start 
./install.sh
following input needed
user = sonarr
group = dietpi
Continue if you are ok. Script should complete in 1-2 minutes. Once done, some cleanup steps needed
systemctl stop sonarr.service
dietpi-software uninstall 150
apt remove sonarr
rm /etc/apt/sources.list.d/sonarr.list
rm /etc/apt/trusted.gpg.d/dietpi-sonarr.gpg
rm Sonarr.main* install.sh
systemctl start sonarr.service
Now you should be able to reach Sonarr as usuall.
!!! This might not be working on ARMv6 !!!