Why does the 4G go away when you enable the Wi-Fi?

I just recently; well my mom just recently bought me an iPhone 4s and I had to enable to Wi-Fi to connect to our router in order to download the Madden 25 Game app and it as successful. But as it successfully connected I notice the 46 icon disappeared. I'm about to head off to school in a few but I want to know if the 4g symbol will reappear when I go on the bus of underground in the subway. Because, I know for a fact that it will lose connection from my router when I walk out of the house. Will it sync with anything else? Or will it re-enable the 4g feature? Why does this happen and how can I bring back the 4g feat because if It synced to my school internet, I think it will block me off from sites like facebook, instagram, etc because that happened a lot when I used to sync my ol' PSP to the school internet back in middle school. Please explain.

The reason why your phone does that is to conserve battery power. My phone does that too. When there's Wi-Fi, the data network icon goes away. It comes back once I turn off my phone's Wi-Fi, or when I get disconnected from a Wi-Fi network. So I'm sure yours will do likewise.

Yes. The phone prioritizes Wi-Fi wherever available. If it is no longer available then the fastest data connection available will take over, in your case 4G.

If you are connected to your school's Wi-Fi & you don't want to use it, simply take it off in the settings & it would switch back to the 4G or if you don't want to use it at all check the Wi-Fi settings & have the phone forget the connection.