你的句子也可以加上to，写成：

After four days in a hospital, John was fired. And all he had done was to go outside and get the license number.

在英语中，当 all sb do作主语时，其后用作表语的不定式可以省略 to，也可以不省略 to。又如：

All I do is (to) type letters. 我的工作是打印信件。

All he does is (to) complain. 他只会发牢骚。

All she does is (to) talk about her boyfriend. 她就不停地讲她的男朋友。

All he did was (to) open the door. 他所做的只是把门打开。

All I did was (to) press this button.我所做的只是按这个电钮。

All I did was (to) touch the window, and it broke. 我只不过碰了一下窗户，玻璃就碎了。

All I had done was (to) give him a little push. 我所做的不过是轻轻地推了他一下。