Self-leveling indicates the difference from the level of the laser housing for the internal leveling mechanisms allowing the device to level itself adequately. Instead of having to waste a lot of time trying to adjust the level to the perfect position, this feature does that for you. If you read "self-leveling with 4 degrees", that would mean that the device can create the level beam within the 4 degrees range. Surpassing 4 degrees, the laser level won't be able to do this and will signal the user to indicate that it is outside of the self-leveling range.