
you don't need to sudo if you're root - if you're using bt4, presumably, you're root, unless you added a new privileged user. just a quickie fyi
running your virtual machine with bridged only shares your host ip with the virtual machine. having deleted the other ifaces in your /etc/network/interfaces file except wlan0, should be the root cause of your networking not working (get it? networking/notworking?). *thinking, thinking, thinking... uhhh... right!* and as far as your wlan0 goes - unless you have a usb wireless device connected to your host and passed through to your vm, it ain't gon' work.
so... what you now need to do is add all your interfaces back, or at least eth0, in order to get your bridging working again. also, going the nat route might be better than bridged. some routers and firewalls don't like that. oh, and also make sure you're running
wicd before starting
wicd-client
i'd go with what brtw2003 has mentioned, and get familiar with networking. it'll not only help you understand the basics, but if you're serious about using bt4, you'll be able to understand networking concepts and use backtrack a lot better.