After using jekyll serve on one machine, a WEBrick server is set up and the site can be accessed from localhost:4000 on this particular PC.
However, I'm wondering how to access this web server from other machines in the LAN, especially for mobile devices? I'm trying to test the jekyll site on mobile devices before pushing the code to Github.
Try jekyll serve --host=0.0.0.0 when you invoke Jekyll on the command line.
That will make Jekyll's HTTP server bind to all available IPs, rather than just to localhost.
You can also add this to your _config.yml with host: 0.0.0.0. GitHub will simply ignore this when you push, so it's safe to use if you don't mind having your work openly accessible on your network.
Without --host=0.0.0.0 Jekyll will output something like this when you start up:
$ jekyll serve
[...]
Server address: http://127.0.0.1:4000/
Server running... press ctrl-c to stop.
But with --host=0.0.0.0 (or host: 0.0.0.0 in _config.yml) you'll notice that it's listening on all interfaces (represented by 0.0.0.0) rather than just listening on the loopback interface (represented by 127.0.0.1)
$ jekyll serve --host=0.0.0.0
[...]
Server address: http://0.0.0.0:4000/
Server running... press ctrl-c to stop.
If you still cannot access your server then there might be a firewall stopping it. Temporarily disable your firewall, or add a port forwarding rule for port 4000.
Once Jekyll is appropriately listening on all interfaces, you can access this from your mobile device using your LAN IP address (retrieved from something like ifconfig or ipconfig depending on your operating system).