What are the rules and constraints governing ATC speed control instructions issued by Approach controllers to arriving jet aircraft on intermediate and final approach?

1. Speed adjustments should not be applied within 4 NM of the runway threshold, should not require speeds below the aircraft's minimum clean/maneuvering speed, and adjustments should normally be expressed in multiples of 10 knots (e.g., 'REDUCE SPEED TO 160 KNOTS').
2. Controllers may order aircraft to fly 50 knots below stall speed.
3. Speed control can be applied right down to the touchdown flare.
4. Speed adjustments are expressed in miles per hour only.