To “burn” a .iso or .img file to a USB device, type:
sudo dd if=bt4-pre-final.iso of=/dev/disk1 bs=1m
where /dev/disk1 is your particular disk. In OS X you can find out by going to the Disk Utility and selecting information on your target drive. In linux, type `dmesg` and that should give you the drive name.
I will be editing this to add more stuff.