You can put the #4 iron shaft in a #5 iron head and cut to 5 iron length. The shaft will play softer because the head weight of the #5 iron is heavier than the #4 iron head. I am assuming the head weights progress in weight in the traditional 7 gram increment. If so, the #4 iron shaft would play approx. 1/3 flex softer in the #5 iron head and cut to the 5 iron length.