Before you download it, why dont you consider purchasing a Raffle Ticket from the Kansas City HackerSpace (CCCKC). You will have a pretty good chance at winning a brand-new MACBOOK! (Only selling 200 tickets!!!)
Seriously, help them out -- they need $$ for chairs, and tools to fill up their hackerspace in a CAVE.
Ok, so..... did you buy one? Good, You can download BackTrack4 here.
Make sure to check the Back|Track blog to see if there is any updated installation/Usage notes before using!
To make a bootable USB drive, install and use unetbootin.
This will make your thumbdrive bootable -- then simply just boot off of that. You can use the steps outlined in the "Installing Backtrack3 on an EEE PC" to install this to your HDD if you want to.
UPDATE: From the Back|Track Blog: We've set up a quick install guide for BT4, as there is no official GUI installer yet (we're working on it).
Remember to look here for basic commands / tips and tricks for getting up and running with BT4.
Oh.. Also took some Shmoocon Party Pics!