The obvious issue is the swingweight. the 5 iron is progressively lighter than the 6 and the 4 is progressively lighter than the 5, so the swingweights would be significantly lighter. Nothing wrong with that if you can feel the head and you hit them OK, but my guess is that they would feel light compared to the 6 iron. 5 would be approx 6-7 swingweights lighter and the 4 would be around 9-10 swingweights lighter. For every 1/2 inch shorter you make a club, you might expect the clubhead speed to go down slightly. Generally 1″ decrease in length will reduce clubhead speed by 1mph, but that is more on the driver side than in irons. Your probably looking at less than 1mph reduction in clubhead speed, everything else being equal. 1 mph in clubhead speed is equal to 2.3 yards. If you add lead tape to the head to get the swingweight up (it would be a lot of lead tape), you would be getting the head weights back to what they would need to be to make the shaft play true to flex. If you did not add the weight back, the shaft will play stiffer with the lighter head weights of the #4 and #5 irons. Suggestion might be to progress the length only 1/4″ between the 6 and 5, then the 5 and 4. Still would need to experiment with the feel, but it would not be so dramatic a difference.