By cutting the gears he means the seating area of the dogs. They can be machined so the hold in contact better with torque. As for the fourth gear well fourth and fifth mesh by dog on dog not dogs in a female drive gear, the newer billet one act more like the female in mesh with the male fifth gear dogs.
note circled in Yellow this is 4th and 5th
see the dog on dog
now look at the new fourth gear
Now do not go thinking you have junk gears not many have broken fourth and like all bikes things can be broken or say shift forks can be forced so hard they bend.