Six in the fourth place means:
Keeping his trunk still.
As has been pointed out above in the comment on the Judgment, keeping the back at rest means forgetting the ego. This is the highest stage of rest. Here this stage has not yet been reached: the individual in this instance, though able to keep the ego, with its thoughts and impulses, in a state of rest, is not yet quite liberated from its dominance. Nonetheless, keeping the heart at rest is an important function, leading in the end to the complete elimination of egotistic drives. Even though at this point one does not yet remain free from all the dangers of doubt and unrest, this frame of mind is not a mistake, as it leads ultimately to that other, higher level.
Translator of this version of the I Ching is Richard Wilhelm. If you missed any posts in this series, please utilize the I Ching label below.