if I plugged in the 4TB USB external drive and added it to the server as a backup drive without letting the server format it, it will be detected as a 4TB drive and the server will be able to read and utilize the entire drive?
By default the 3TB and 4TB drives are setup as GPT drives because the usual 2TB and smaller drives are setup MBR. This is a mathimatical limit to MBR, you can't have a MBR drive larger that 2.1TB.
So then you have to setup up the larger drives as GPT. However with WHSv1, when you format the 3 or 4TB drives, you don't have the option in the wizard to set them up as GPT. So you do this on your PC. But as I wrote, you do have to add the drive as a backup but it gives you the option not to format, or else you would be out of luck. If you did format the drive, then it would format it as MBR and you would end up with a 2.1TB drive and 768GB of unused space on a 3TB drive. The same would be if you tried to add the drive as a pool drive.
I have a esata enclosure with 2 3TB drives as backup on my EX470 so I know that it works.
There will not be an issue with the backups and a 2TB limit, that limit is with WHS2011. The only issue you will have is when the drive gets full, if you decide to get a 2nd USB 4TB drive, then you can split the shared folders backups between drives.
Sorry for the long explanations, but that's me.