Nginx

The internet shouldn't talk to Express on :4000 directly. Nginx takes 80/443 and quietly proxies inward.

server {
    listen 80;
    server_name api.yourdomain.com;

    location / {
        proxy_pass http://127.0.0.1:4000;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }
}

sudo ln -s /etc/nginx/sites-available/club-api /etc/nginx/sites-enabled/
sudo nginx -t && sudo systemctl reload nginx

nginx -t before reload — saves you from locking yourself out of a bad config.