How does the playabilty of a fairway wood change when it is cut down dramatically ie: 4 inches?
The playability of the head does not change at all, unless you add a bunch of weight to it. Even then, mass and dimensional characteristics will not change much. Fairway woods, by the nature of their design, have high playability. The challenge when cutting down a club as much as you indicate is maintaining the feel of the club and to some extent the shaft dynamic. When you cut the shaft down from the butt end, you are cutting the largest diameter of the shaft, the strongest part of the shaft, so the shaft may play or feel a little weaker, especially if you add weight back into the head someway to try and maintain some head feel. Obviously if you cut it down and do nothing to the head weight, the swing weight (and thus the head feel) will be lighter. Whether any of this matters is totally up to the person hitting it and how it performs for them at the shortened length. If this is for a junior, for example, it is much more important for their development that the length of the club fit them than what the swing weight is. This is not only true for juniors, but any developing player. Again, if the person does not notice a loss of head feel, then it’s not an issue. Hit it an evaluate if any head weight adjustments need to be made.