You don't give up much velocity either since it's documented at @ 35fps/inch, so you lose 70fps to the 6" barrel and that's no big deal.
I have serious doubts about those numbers - unless they specifically refer to a 38, not a 357.
4" is too short to take full advantage of a 357. Loads not specifically developed for a short barrel should show quite significant improvement.
I have a Marlin, so I looked into that - some loads with madium weight bullets (158) out of 18 inch barrel have more energy at 100 yards than at muzzle from 4" - the gain is that great.
6" is not good for carry, but neither, I discovered, is 4" - at least not under-arm or IWB for a medium-height person like me. 3" is perfect for that.
But as a hunting, range, HD or car gun that is not going to be carried, there are only advantages and no disadvantages to having a 6"
Speaking of, is there a special wisdom to a car-gun load?
I would use a heavier FMJ bullet for such, not a 125gr HP I'd use for carry. Instant incapacitation is less important - a person outside a car, especially in another car is not likely to jump and grab you if he is not incapacitated immediately. A highjacker inside your car will likely be shot with a muzzle pressed into his flesh or close to that - with 20 grains worth of gasses augmenting the action of the bullet.
On the other hand, shooting at another car, penetration is more important. From the notes of the argentine, having to defent oneself from attack from another car is a realistic scenario - of course he advises a semi-auto for that due to capacity considerations.
miko