Once I am connected to a Wi-Fi network (regardless of my other connections to 3G and 4G) is there any way to guarantee that all my data goes through Wi-Fi?
Though @Matthew answer is correct that a phone will default to Wi-Fi when it is available, that doesn't always ease the mind.
Something you can do to allay your concern is switch the phone into Airplane Mode. This will disable the cell signal and hence all data travelling through your carrier. However, it does not disable Wi-Fi.
From the main screen, press your Power Button (located on top) Select Airplane Mode to disable mobile network signal
Of course, you won't be able to receive any calls, and this might not be what you want, but it will certainly allay your concerns of using data.
Your phone will default to WiFi when it's available. If you're paranoid you can disable data while using WiFi but it's not necessary. I'm not sure how to do it on the Bionic but you could see if it's in the menu that pops up when you hold the power button for a couple seconds or something like unchecking Use packet data in
Settings -> Wireless and network -> Mobile networks.