It is not as good as some of the other barrels out there but only applies if you do a heck of a lot of shooting. For the average person, it's a none issue.
4140 has been used for decades + for firearms barrels and has been more than up to the task. If I liked a firearm enough to buy it, 4140 would not be i disqualifier.
Jim
I always like to think of how much money I will have spent on ammo in order to wear out a barrel. Hint: It's *way* more than the cost of a $200 - $300 barrel.
A 4150 steel barrel has a slightly higher carbon content making the steel slightly harder than that of a 4140 barrel. Ideally a 4150 barrel should last longer and stand up to a little more abuse, but the average shooter will do just fine with the 4140 barrel.
I get the impression that many here don't know what "Nitriding" does to steel. S&W Nitrides the barrels (inside and out) causing the steel to be extremely hard and rust-proof. Discussions of barrel steels are pretty much moot when barrels are Nitrided.
Machine gun barrels are Nitrided for durability.
Do yourself a favor and look up Nitrocarburizing or Nitriding on Wikipedia.