Apache无法连接PM2托管的Node.js进程问题求助

各位大佬好，我在Ubuntu 20上部署了Apache，最近一直遇到连接错误，在/var/log/apache2/error.log里看到如下日志：

[proxy:error] [pid 7064] (111)Connection refused: AH00957: http: attempt to connect to 127.0.0.1:4000 (127.0.0.1) failed

[proxy_http:error] [pid 7064] [client ...] AH01114: HTTP: failed to make connection to backend: 127.0.0.1

我的虚拟主机配置是这样的：

<IfModule mod_ssl.c>
<VirtualHost *:443>
ServerAdmin me@example.com
ServerName example.com
ServerAlias www.example.com
DocumentRoot /var/www/example.com
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined

RewriteEngine on

# Reverse Proxy Stuff for Node
ProxyPreserveHost On
ProxyPass / http://127.0.0.1:4000/ Keepalive=On
ProxyPassReverse / http://127.0.0.1:4000/ Keepalive=On

SSLCertificateFile /etc/letsencrypt/live/example.com/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/example.com/privkey.pem
Include /etc/letsencrypt/options-ssl-apache.conf
</VirtualHost>
</IfModule>

我在网站目录/var/www/example.com下用PM2启动了Node.js应用，命令是：
pm2 start index.js -p 4000

执行pm2 status显示进程是在线状态，而且PM2日志里也没有任何错误，但Apache和PM2就是没法正常通信。

用netstat -tulpn检查后发现，根本没有进程在监听4000端口，我实在找不到问题出在哪了，有没有大佬能给点思路？

备注：内容来源于stack exchange，提问作者Clifton Labrum