Unless the "fourth cog" is coincidentally worn on both freewheels, which is possible, that is weird. Shifting to the larger chainring increases chain tension and reorients the rear derailleur's jockey wheel, and, with Simplex, the rear derailleur body's position. Does the derailleur hang plumb and true? Are there any bends or other defects in the pulley cage?
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069