502 Error with Apache2 reverse proxy on Ubuntu 14.04 - node.js

Question

I'm trying to set up a reverse proxy with an Apache2 server accepting requests at port 443 (to enable https). The idea is to have the Apache2 server accept the request and forward it on to my Node server, which is listening on port 4443. I've looked through several example configurations for setting up the virtual hosts, and after toggling a few options I still get the 502 error. Both servers are hosted on a DigitalOcean droplet running Ubuntu 14.04.
I can hit port 4443 directly in Chrome and it serves the Node app. When I load the site without a port number, it loads port 443 (as expected) and I get this error:
Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /.
Reason: Error reading from remote server
Apache/2.4.7 (Ubuntu) Server at [domain.com] Port 443
In the console, it says:
Failed to load resource: the server responded with a status of 502 (Proxy Error)
Here are the resources I'm following:
https://www.digitalocean.com/community/tutorials/how-to-use-apache-http-server-as-reverse-proxy-using-mod_proxy-extension
https://support.mayfirst.org/wiki/how-to/servers/configure-nodejs-with-apache
This is my config file. The actual IP and domain are replaced with [IP] and [domain.com].
<IfModule mod_ssl.c>
<VirtualHost *:443>
# The ServerName directive sets the request scheme, hostname and port that
# the server uses to identify itself. This is used when creating
# redirection URLs. In the context of virtual hosts, the ServerName
# specifies what hostname must appear in the request's Host: header to
# match this virtual host. For the default virtual host (this file) this
# value is not decisive as it is used as a last resort host regardless.
# However, you must set it for any further virtual host explicitly.
#ServerName www.example.com
ServerAdmin admin#[domain.com]
ServerName [domain.com]
ServerAlias www.[domain.com]
DocumentRoot /var/www/[domain.com]/public_html
#SSL Configuration
SSLEngine On
SSLProxyEngine On
SSLCertificateFile /etc/letsencrypt/live/[domain.com]/cert.pem
SSLCertificateKeyFile /etc/letsencrypt/live/[domain.com]/privkey.pem
Include /etc/letsencrypt/options-ssl-apache.conf
SSLCertificateChainFile /etc/letsencrypt/live/[domain.com]/chain.pem
ProxyPreserveHost On
ProxyRequests off
ProxyPass / http://[IP]:4443/ retry=1 acquire=3000 timeout=3000 Keepalive=On
ProxyPassReverse / http://[IP]:4443/
# Available loglevels: trace8, ..., trace1, debug, info, notice, warn,
# error, crit, alert, emerg.
# It is also possible to configure the loglevel for particular
# modules, e.g.
#LogLevel info ssl:warn
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
# For most configuration files from conf-available/, which are
# enabled or disabled at a global level, it is possible to
# include a line for only one particular virtual host. For example the
# following line enables the CGI configuration for this host only
# after it has been globally disabled with "a2disconf".
#Include conf-available/serve-cgi-bin.conf
</VirtualHost>
# vim: syntax=apache ts=4 sw=4 sts=4 sr noet
</IfModule>
How can I edit my config file to resolve this 502 error?