4" is just the SAAMI standard pressure/velocity test barrel length for this chambering. All commercial ammunition velocities are reported for SAAMI standard test barrel lengths except as noted, and for the few where there is more than one SAAMI standard test barrel length (some pistol rounds, particularly where rifles are made for them as well) then which test barrel length was used should be specified.
In your case you should see about 4½% to 6½% increased velocity over a 4" barrel, depending on bullet and powder used. That's small enough that gun-to-gun chamber tolerances can make as much difference in some instances, so there's no guarantee you'll actually realize the increase as compared to the test barrel velocity reported. It's just the difference you would see if you cut your own barrel down to 4" after testing at 5". As the others have said, there's no effect on peak pressure.
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