Yesterday, the best answer is: 4^x.  (Contributed by asdf. Good Job!)
asdf solved the puzzle at 00:38:08.715; length: 4
dnialh solved the puzzle at 00:38:17.885; length: 4
Collatz solved the puzzle at 02:06:59.670; length: 4
jabrams solved the puzzle at 17:28:26.885; length: 4