unless the listed GVW for the trailer is 16.000lbs, a 12,000 lb trailer with 4000lb carry is still only a 12.000lb trailer.... 
 as for the 16000lb rating of a hitch, this is not the "fail" rating of the hitch, but is the full usable rating. 
 when built hitches are designed with a minimum of a 3:1 safety factor. 
  this means that a 16,000lb hitch will only fail after it gets in the neighborhood of 48,000 lbs.
 and not only are the hitches engineered with a minimum 3:1 safety factor, they must be tested to carry the rating.
  I agree with the law that there is plenty of safety factor built in to pull a full 16,000lb trailer with a 16,000lb hitch.
 even with a 10,000lb hitch, the tongue will be torn out of the trailer before the 5th wheel hitch fails..