That code removed 4GB from every members account which is what you initially asked for 4GB.
If you wanting to remove TB then just change the amount 1TB = 1099511627776.
If your wanting just to remove from those specific accounts use something like
Code:
UPDATE users SET uploaded = uploaded - 1099511627776 WHERE username = "Doniukas"
Note the above code will REMOVE 1 TB from Doniukas account, all you have to do is change the username to whatever you want and change the amount to remove.