According to the ammo oracle, there is no performance benefit using .357 magnum over .38 +P when shooting a snub nosed revolver with a barrel of 2" or less. 

What it doesn't discuss is a gun with a barrel length longer than 2". Is there a substantial performance benefit for a .357 magnum over a .38 +P when fired from a gun with a barrel length of 4"?

If so, then what is the best .357 magnum round for a revolver with a 4" or longer barrel?

I think there is a big difference even in a 2" barrel.

There is a significant difference in performance in all barrel lengths.  There is even a more significant difference in felt recoil.  357 out of a lightweight snub is extreme and much more difficult to control than 38sp.  It is all trade offs and compromises.  It is probably safe to say, for the average person, the cons outweigh the pros in a small lightweight revolver.  In a larger heavier revolver, the pros may outweigh the cons, depending on the individual and his proficiency, etc.

Depends upon the target.

If shooting ground hogs, those 125 grain JHPs are hard to beat.

If shooting grizzly bears, a heavy, hard cast Keith type bullet is hard to beat.

If shooting paper, then a mid range 38 wad cutter is hard to beat.

I keep a 6" model 27-2 loaded with 145 grain Silvertips.  Trouble is, the 1980s called, and they want their ammunition back.

"If so, then what is the best .357 magnum round for a revolver with a 4" or longer barrel?"
I haven't stayed current on revolver ammo but, the Federal 125 gr. JHP used to be the go to self-
defense round in the .357...
In my snub, I keep the old FBI load. 38 spl. 158gr. +P  lead hollow point.

In this link Buffalo bore lists the velocities in real world guns at the bottom on the page:

https://www.buffalobore.com/index.php?l=product_detail&p=103

Out of a 4 inch, 125gr jhp at 1603fps

.38 special +p is listed here:

https://www.buffalobore.com/index.php?l=product_detail&p=109

Out of 4 inch, 1258fps


There seems to be a lot of street data suggesting .357 mag in a 4in barrel is superior for whatever reason. The 2in sounds hard to shoot to me.

A 4" .357 is a whole other animal than a 2" or any length .38spl

Its definitely a very powerful round.  Possibly the most powerful round that is still practical to use along with 10mm IMO.

This is what I have always used, except for hunting and then it was 158 gr in 357.
I like wadcutters for simple target practice

125 gr 38 plus p is supposed to be hell on a k frame

I have shot Hornady .357 125 grain critical defense out of my 2" 640 with uncle mikes grips and didn't find it unpleasant. They make a mess out of rattlers.

I have also shot a few heavy .357 handloads with 173 grain hard cast bullets out of the same gun just to see how it handled them. I don't recommend it as a general practice in a J frame but in a pinch it can be done.

I've shot American Eagle .357 magnums out of my 640, just to see what it felt like. I didn't feel that it was significantly uncontrollable or painful. 

Shooting .38s out of my full size .357 magnums is less unpleasant that shooting a Glock 19...and I like shooting my Glock 19. 

However, the data presented in the ammo oracle, data from Dr. Fackler, states that .357 magnums out of a 2" barrel are do not provide substantially better terminal performance over .38 +P

Yeah snub noise 38 special, and full size 357 magnum revolver go 4" or longer. It's kind of like a 308 out of a short 8" barrel is pointless. Better off using 7.62x39mm instead. Short barrels don't allow for the full powder burn of these stronger rounds, so you lose all the potential and instead just get a big fireball and boom. 

357 Magnum is significantly faster out of a 2 inch barrel than any 38 Special +P out of a 2 inch barrel when comparing similar bullet weights assuming both cartridge are loaded close to SAAMI MAP.  Those ballistics are easy to measure.  http://www.ballisticsbytheinch.com/ will show this fairly well.  35,000 psi of 357 Magnum makes is significantly more potential than only 20,000 psi of 38 Special +P

That said there is a lot of arguments on whether the increase velocity of 357 Magnum make it more effective in a self defense situation especially in the case of snub nose revolvers where both cartridges give up a lot of their potential.  The shoot-ability of the snub nose likely plays a significant role in there ability to be employed effectively also beyond simple balistics.

Before I bought my S&W 442 I borrowed a friend's 340PD and after shooting a few 357 Magnums loads through that I quickly realized that an air-weight 357 Magnum was not worth a damn to me.  The muzzle blast and painful recoil made it pointless compared to how much better and faster I could shoot a similar weight 38 Special.  The extra cost of the gun compared to the S&W 442 I ended up with made the decision easy.  Not to mention 38 Special +P performs better in a 38 Special chamber than a 357 Magnum chamber and you need all the help you can get in the short guns.

I always suggest that anyone thinking about buying a 357 Magnum snub nose, especially the air-weight models to go borrow/rent one before you lay down the significant money.  Then race to six rounds.  Shoot five 38 Special +P as fast as you can and do a reload fire a sixth round.  Compare that time to doing the same thing with 357 Magnum.  See if you fingers will even let you get the cylinder open to do the reload.  Not many shooters can shoot a 357 Magnum snub nose revolvers well enough to justify carrying them.

Yes there is a significance between the two cartridges.  Out of a snub nose it's an easy hands down choice for me.  I went  with a S&W 442 because like a previous poster I shot a friends 340PD and after 5 rounds of Remington 125gr Scalloped jacket hollow points I couldn't feel my ring and pinky finger.  It took  like two weeks before the numbness went away.  With 38+p from the 442 it's no where near as bad as the 340PD.  

Out of a 4" 357 Mag I like the Remington SJHP which is like the Federal load of the same weight.  Both were widely used back in the day when Revolvers were on LEO's hips.
When i'm out of town and in the sticks I usually carry a larger caliber revolver but when I do carry the .357 I have it loaded with 158gr or 180gr cast WFN bullets.  

 
For my S&W 442 I tried several modern +p loads in the 125gr-130gr varieties.  None of them would shoot to the sights so I again went old school with the Federal 158gr +p FBI load.  I also tested the Remington load of the same weight.  The Remington seems to shoot better from my 442.  They expanded and penetrated well in the wet news print that I tested them in.

Here is a good resource with gel tests. 38 & 357 mag in 2" & 4" barrel.

https://www.luckygunner.com/labs/revolver-ballistics-test/

ETA I like 158 gr XTP in magnum loads. Half inch expansion and penetration over 20" in either barrel. I like 357 magnum for penetration.

Hornady critical defense 125 gr .357 mag was advertised as 1200 fps 400 ft-lb and low flash out of a 2 1/8 in j frame.
Otherwise I use the .38 special +p.

Dr. Fackler is correct about the terminal performance being the same. If a hollow point from either caliber penetrates in the 12 to 18 inch window, it will do the exact same thing on a human body in regards to performance. That is expand to a certain diameter and create a wound track in the body of the same expanded diameter until it stops.

What you gain with the faster .357 is better hard barrier penetration due to the speed and deeper penetration if using solid ammo that doesn't expand in soft tissue on thicker animals.

And in those cases, I'm not going to be carrying a snub revolver.  Hell, I'm not carrying a revolver unless I'm hunting/outdoors.

In a snub, I'll just use .38 with a snub optimized defense round.  If that's what I am carrying, I'm not really concerned about the extremes in defense situations otherwise I'd have brought more firepower.

This argument is sort of like 9mm vs. 357sig vs. .40 vs. .45 vs. 10mm.

I'm not going to give up faster shooting, more ammo, and less recoil for more recoil, less capacity, and slower shooting unless I need the extra power.  And then I'm going to carry a weapon optimized to take advantage of that power that I'm hauling around.

I agree. Which is why I carry the 9mm vs the .357 Sig. The extra flash, recoil, and concussion while indoors isn't worth the fact that I can shoot the 9mm with greater accuracy due to those factors being a lot less.