Jeremy Hellickson or Tanner Roark are the ideal 4/5 starters.
From 2014-2018 Tanner Roark had the 42nd most SP WAR in MLB and had 4 out of 5 seasons where he put up 2 WAR or better. He was an "ideal 4/5 starter" on a perennial playoff team that usually had one of the stronger rotations in the game. He also would have been an "ideal 1/2 starter" on 6-8 teams last season.
I don't really disagree about Hellickson, but when I say, "so in reality a #4/5 starter is basically worthless" what I mean is a guy like Hellickson gets a 1/$1.3 million contract, 1200 innings into his career. That's basically worthless in a league where you pay a minor league callup $600K.