diff --git a/fluent-package/apt/systemd-test/downgrade-to-v4.sh b/fluent-package/apt/systemd-test/downgrade-to-v4.sh
new file mode 100755
index 000000000..96b962d3c
--- /dev/null
+++ b/fluent-package/apt/systemd-test/downgrade-to-v4.sh
@@ -0,0 +1,79 @@
+#!/bin/bash
+
+set -exu
+
+. $(dirname $0)/../commonvar.sh
+
+# Install v4
+sudo apt install -y curl ca-certificates
+curl -fsSL https://toolbelt.treasuredata.com/sh/install-${distribution}-${code_name}-td-agent4.sh | sh
+
+# td-agent is running
+systemctl status --no-pager td-agent
+
+# fluent-package 5 (LTS)
+#curl -fsSL https://toolbelt.treasuredata.com/sh/install-${distribution}-${code_name}-fluent-package5-lts.sh | sh
+#sudo apt purge -y fluent-lts-apt-source
+
+# Ensure to install the current
+sudo apt install -V -y \
+    /host/${distribution}/pool/${code_name}/${channel}/*/*/fluent-package_*_${architecture}.deb
+
+# td-agent.service is already masked (link to /dev/null), and remove td-agent.service alias not to conflict with v4
+sudo systemctl unmask td-agent
+
+# Keep log and .conf and so on. dpkg reports "rc fluent-package" and "rc td-agent" status.
+sudo apt remove -y fluent-package td-agent
+
+# fluentd.service is already masked (link to /dev/null), then remove it.
+sudo systemctl unmask fluentd
+
+# Drop symbolic links and recreate real directory.
+sudo rm -f /var/log/td-agent
+sudo rm -f /etc/td-agent
+sudo mkdir -p /var/log/td-agent
+sudo chown td-agent:td-agent /var/log/td-agent
+
+# Migrate logs
+sudo mv /var/log/fluent/* /var/log/td-agent/
+
+# Reinstall v4, but /etc/td-agent is empty.
+curl -fsSL https://toolbelt.treasuredata.com/sh/install-${distribution}-${code_name}-td-agent4.sh | sh
+sudo mv /etc/fluent/td-agent.conf /etc/td-agent/
+
+sudo systemctl enable td-agent
+sudo systemctl start td-agent
+
+# Test: service status
+(! systemctl status --no-pager fluentd)
+(systemctl status --no-pager td-agent)
+
+# Test: config migration
+test -d /etc/td-agent
+(! test -h /etc/td-agent/fluentd.conf)
+test -e /etc/td-agent/td-agent.conf
+
+# Test: log file migration
+test -d /var/log/td-agent
+test -e /var/log/td-agent/td-agent.log
+
+# Test: bin file migration
+test -x /usr/sbin/td-agent
+test -x /usr/sbin/td-agent-gem
+
+# Test: environmental variables
+pid=$(systemctl show td-agent --property=MainPID --value)
+env_vars=$(sudo sed -e 's/\x0/\n/g' /proc/$pid/environ)
+test $(eval $env_vars && echo $HOME) = "/var/lib/td-agent"
+test $(eval $env_vars && echo $LOGNAME) = "td-agent"
+test $(eval $env_vars && echo $USER) = "td-agent"
+test $(eval $env_vars && echo $FLUENT_CONF) = "/etc/td-agent/td-agent.conf"
+test $(eval $env_vars && echo $TD_AGENT_LOG_FILE) = "/var/log/td-agent/td-agent.log"
+test $(eval $env_vars && echo $FLUENT_PLUGIN) = "/etc/td-agent/plugin"
+test $(eval $env_vars && echo $FLUENT_SOCKET) = "/var/run/td-agent/td-agent.sock"
+
+# Test: No error logs
+# (v4 default config outputs 'warn' log, so we should check only 'error' and 'fatal' logs)
+sleep 3
+test -e /var/log/td-agent/td-agent.log
+(! grep -e '\[error\]' -e '\[fatal\]' /var/log/td-agent/td-agent.log)
diff --git a/fluent-package/apt/systemd-test/test.sh b/fluent-package/apt/systemd-test/test.sh
index 67c98b115..247cbaf51 100755
--- a/fluent-package/apt/systemd-test/test.sh
+++ b/fluent-package/apt/systemd-test/test.sh
@@ -20,6 +20,7 @@ if [ ! $image = "images:debian/12" ]; then
     test_filenames+=(
         update-from-v4.sh
         update-to-next-version-with-backward-compat-for-v4.sh
+        downgrade-to-v4.sh
     )
 fi
 
diff --git a/fluent-package/yum/systemd-test/downgrade-to-v4.sh b/fluent-package/yum/systemd-test/downgrade-to-v4.sh
new file mode 100755
index 000000000..909e21f07
--- /dev/null
+++ b/fluent-package/yum/systemd-test/downgrade-to-v4.sh
@@ -0,0 +1,97 @@
+#!/bin/bash
+
+set -exu
+
+. $(dirname $0)/commonvar.sh
+
+# Install v4
+case ${distribution} in
+    amazon)
+	curl -fsSL https://toolbelt.treasuredata.com/sh/install-amazon2-td-agent4.sh | sh
+	;;
+    *)
+	curl -fsSL https://toolbelt.treasuredata.com/sh/install-redhat-td-agent4.sh | sh
+	;;
+esac
+
+# Not auto started
+(! systemctl status --no-pager td-agent)
+
+# Start service to generate log
+sudo systemctl enable --now td-agent
+sudo systemctl stop td-agent
+
+# Install the current
+sudo $DNF install -y \
+    /host/${distribution}/${DISTRIBUTION_VERSION}/x86_64/Packages/fluent-package-[0-9]*.rpm
+
+sudo systemctl daemon-reload
+sudo systemctl enable --now fluentd
+sudo systemctl stop fluentd
+
+# Remove td-agent.service alias not to conflict with v4
+sudo systemctl disable td-agent
+
+sudo $DNF remove -y fluent-package
+
+sudo rm -f /var/log/td-agent
+sudo rm -f /etc/td-agent
+sudo mkdir -p /var/log/td-agent
+sudo chown td-agent:td-agent /var/log/td-agent
+
+sudo mv /var/log/fluent/* /var/log/td-agent/
+
+# Downgrade to v4
+case ${distribution} in
+    amazon)
+	curl -fsSL https://toolbelt.treasuredata.com/sh/install-amazon2-td-agent4.sh | sh
+	;;
+    *)
+	curl -fsSL https://toolbelt.treasuredata.com/sh/install-redhat-td-agent4.sh | sh
+	;;
+esac
+
+sudo systemctl daemon-reload
+sudo systemctl enable --now td-agent
+
+# Test: service status
+(! systemctl status --no-pager fluentd)
+(systemctl status --no-pager td-agent)
+
+# Test: config migration
+test -d /etc/td-agent
+(! test -h /etc/td-agent/fluentd.conf)
+test -e /etc/td-agent/td-agent.conf
+
+# Test: log file migration
+test -d /var/log/td-agent
+test -e /var/log/td-agent/td-agent.log
+
+# Test: bin file migration
+test -x /usr/sbin/td-agent
+test -x /usr/sbin/td-agent-gem
+
+# Test: environmental variables
+case ${distribution} in
+    centos)
+	# --value is not supported yet
+	pid=$(systemctl show td-agent --property=MainPID | cut -d'=' -f2)
+	;;
+    *)
+	pid=$(systemctl show td-agent --property=MainPID --value)
+	;;
+esac
+env_vars=$(sudo sed -e 's/\x0/\n/g' /proc/$pid/environ)
+test $(eval $env_vars && echo $HOME) = "/var/lib/td-agent"
+test $(eval $env_vars && echo $LOGNAME) = "td-agent"
+test $(eval $env_vars && echo $USER) = "td-agent"
+test $(eval $env_vars && echo $FLUENT_CONF) = "/etc/td-agent/td-agent.conf"
+test $(eval $env_vars && echo $TD_AGENT_LOG_FILE) = "/var/log/td-agent/td-agent.log"
+test $(eval $env_vars && echo $FLUENT_PLUGIN) = "/etc/td-agent/plugin"
+test $(eval $env_vars && echo $FLUENT_SOCKET) = "/var/run/td-agent/td-agent.sock"
+
+# Test: No error logs
+# (v4 default config outputs 'warn' log, so we should check only 'error' and 'fatal' logs)
+sleep 3
+test -e /var/log/td-agent/td-agent.log
+(! grep -e '\[error\]' -e '\[fatal\]' /var/log/td-agent/td-agent.log)
diff --git a/fluent-package/yum/systemd-test/test.sh b/fluent-package/yum/systemd-test/test.sh
index 7ccd2b56c..6e1a35e84 100755
--- a/fluent-package/yum/systemd-test/test.sh
+++ b/fluent-package/yum/systemd-test/test.sh
@@ -20,6 +20,7 @@ if [ ! $image = "images:amazonlinux/2023" ]; then
     test_filenames+=(
         update-from-v4.sh
         update-to-next-version-with-backward-compat-for-v4.sh
+        downgrade-to-v4.sh
     )
 fi