@iignatov Thanks for reiterating whether I am running things as Administrator…that was the solution! I assumed that since I am a single user with Admin rights to my computer that I would automatically be running the command prompts as Administrator. Turns out this is not the case.
By right-clicking on Windows Command Prompt (on Windows 8.1), selecting ‘Run as Administrator’, and navigating to my project folder and running ionic serve, I was able to run both my beta4 project and the beta6 starter flawlessly (i.e. without any errors whatsoever) - yay! 
I usually use the Node.js command prompt - how do I ensure I’m running that as Administrator? When I right-click on it there is no option to run as Administrator…
Thanks again!