My advice is to proceed carefully. Drastically shortening the factory barrel length will have an effect on handling, balance, muzzle velocity, muzzle blast, recoil, and aesthetics. Lets begin with the ballistic effects. The smaller the bore size and the higher the velocity envelope, the more velocity you loose per inch of barrel length you reduce. The heavier the bullet you shoot, the less velocity you loose by shooting in a shorter barrel, as all things being equal, a smaller the powder charge takes less barrel length to achieve optimum velocity. I came up with a way to resolve these problems when I decided I had to build a 20" .375. My solution was to rechamber from .375 H&H to .375 Ultra (I'd have gone to .378 Weatherby but for the cost of the brass). The 20" Ultra drove a 300 gr bullet at 2600 and a 270 at 2850, just like a long barreled H&H. But I got way ahead of the curve with heavy 350 and 380 gr bullets driving them up to 150 fps faster than was possible in the H&H. If I was building another . . . and I am, I'd go with a 22" barrel.
Now for the effect that shortening the barrel has on the shooter. Blast can become a real problem, again its worse with a small bore. Recoil will change, but whether or not it becomes a problem depends on the individual. The bottom line is that you are reducing gun weight which increases recoil velocity with the same load. Velocity will be reduced in most cases enough that you should confirm your point of impact at a variety of ranges. The result might have an effect on the coincidence of the trajectory and the sight, but its unlikely to effect the terminal performance of the bullet on a live target. The shorter barrel might result in a rifle that is more accurate, being stiffer, and perhaps you'll get a better crown, and perhaps the muzzle will happily coincide with the tightest part of the bore, although that will be accidental if you simply tell your smith to knock off 4" rather than to shorten the barrel to coincide with the tightest part of the bore. Chances are any change in accuracy, be it for better or worse, probably won't have any effect on your ability to hit in the field.
Finally we come to handling and aesthetics. Shortening the barrel will change the balance, and if you go too far, the center of balance shifts too far rearward, resulting in a clubby feel. Where the barrel was shortened to make it quicker, the opposite becomes the realization, with a piece that actually takes longer to get on target and is difficult to hold there. This can be negated by choosing a larger barrel contour, but now you're going the route of a custom rifle, rather than just making your factory rifle better suited to your needs. If the barrel is too short relative to the length of the stock's forend, the appearance is unsatisfactory. If you have to drill down into the length of the forend to pour in lead shot, this will improve the rifle's balance and reduce recoil, but the rifle is now no lighter than before it was shortened. Just remember, everything you do to change the dimensions of that rifle, have a cumulative effect downstream.
