Can you share a Wi-Fi connection using Personal Hotspot? I have an iPhone 4 with iOS 6.1.3. I turn on cellular data + 3G and Wi-Fi constantly.

I was at a site office today and my iPhone automatically connected to the office's Wi-Fi. The tech guy was not around to provide me with the Wi-Fi password to use on my Windows laptop. I then connected my iPhone to my Windows laptop using the USB cable and turned on Personal Hotspot on my iPhone.

At that point, is my laptop using cellular data through iPhone, or the Wi-Fi access point my iPhone is connected to?

I noticed that my iPhone has the 3G symbol in the status bar. But when I disconnect the cable from my laptop, it switches to Wi-Fi.
If you are connecting through the Personal Hotspot on iPhone, it will switch to using the cellular data network that your phone uses.  

This is the nature of how the Personal Hotspot feature is designed. See this article on Apple's website.

So in your case, your laptop was not piggybacking through the wifi connection on iPhone. It's an unfortunate design choice but as far as I know a current limitation. Maybe with a jailbroken device you could get past that but doesn't seem worth it for this case at all.