Transform panel. Quaternion mode. What is the 4L button good for? When you switch the rotation mode to Quaternion in the Transform panel, then you will have four values available. W, X, Y and Z. That's a quaternion, which prevents gimbal lock.
Switching to quaternions also reveals a button called 4L. What it does is to show or hide the lock behind value W. The description of this button is rather cryptic. And i cannot figure out what is meant.
So what is this button good for?
Found it out. A quaternion is made of three axis x, y and z. And the value W, which is a calculated value. Normally this calculated value has no lock since it is a mathematical calculated value. It can be used to retrieve the actual rotation around the defined angle.
The 4L button simply allows you to reveal a lock at the W value too. And that's already it for the functionality of this 4L button.