How does android wifi hotspot work when other wifi is available? Say I have my phone (nexus 4) setup as a wireless hotspot for my tablet to use while outside... Obviously this will make use of my mobile data plan. But when I get near a wifi signal that I regularly use, such as at home, will my phone continue to use my mobile data, or will it use wifi instead, and repeat the wifi signal? That is, do I have to remember to disable the hotspot every time I don't need it? And on that note, how can I make my nexus 7 prefer wifi other than my phone hotspot when available?
When your phone is in hotspot mode, it will only use your mobile data.  You'll need to remember to switch the hotspot off when you have wifi available.

You might be able to script the shutting down of the hotspot with Tasker or Locale, but I'm not sure if the "WiFi near" function would work when in hotspot mode (a good topic for its own question).

Your Nexus 7 will tend to connect to the strongest signal available, and then stay connected to that network until it loses the connection.    When you turn off the hotspot on your Nexus 4, the tablet should auto-connect to an available Wifi network.