Four knocks? Four knocks for what? Who would knock four times, and what did that have to do with the Doctor?

He looked at him, as if waiting for explanation, and stalled for only a moment before nodding and inputting the answer.

"Yes, Sir," he said, and waited for the door to respond. He wanted to ask. He wanted to know. But that could wait.

The door unlocked and he took a step forward, touching his hand out to touch the Doctor's arm. The movement could appear unconscious, but it wasn't. It was precise.