This makes sense, but trying to slightly improve on your #4 with a better#4 means you are not getting a very good pitcher.
Just replacing your 3rd best pitcher knocks your 3 to 4 and your 4 to 5. In theory, adding any pitcher better than your 5th best starter is basically replacing the 5. Id prefer replacing the 5 with a 2 or 3 than a 4, but money and circumstances often keeps that from happening.
Also, when you get to 4’s or 5’s, many are stabs in the dark and almost all of them are hard to predict what they will give the team the following year.
In recent years, we added Kluber, Paxton, Wacha, Hill, Richards, Perez (twice) and Pivetta. To me, it’s not a successful strategy unless it is coupled with adding a 1 or 2.