357 Mag. is a weapon intended for the extra power which it contains. If the shooter does not need the extra 357 power then 38's will do the job. A 6 inch barrel is necessary to allow the powder to burn in, in order to creat the gases sufficient to push the bullet to 357 Mag. specs. Shorter barrels are a waste of powder. Handling and shooting your handgun is a necessary process in order for someone to get confortable and confident with it.
Are you saying that a factory .357 Magnum round fired through a 4" barrel won't be traveling faster and hitting harder than a factory .38 Special of the same bullet through the same barrel? If you're not saying that, how is it a waste of powder to have a harder hitting bullet? I agree that you may not get the maximum possible from the round, but all else being equal, the BG will likely not be thinking, "Geez, if that gun had a 6" barrel, I would be dead already, that 4" barrel was a bad choice. He should have just shot me with a .38 and saved some money."
From the tables I've seen, the .357 Mag beats the delivered energy of a .38 Special even from a 2" barrel and gives you whatever deterrent effect that a louder bang and brighter muzzle flash might offer. In a self-defense situation, shooting a gun you're comfortable with makes it more likely you will hit the target. You won't be wearing hearing protection and you'll feel more recoil than you're used to, but those will most likely go unnoticed. You'll deliver almost twice as much energy into the target and that's what really matters. These charts tell the tale.
That's why I put many more .38s through my bedroom gun than .357s. The gun will last longer and I will be more confident and comfortable in my ability to shoot straight with it. The 2" of barrel length won't matter much inside 15'.