“a direct reaction to the encounters with the Soviet T-34 and KV-1 tanks and against the advice of Wa Prüf 6.[Notes 1][12] The T-34 outclassed the existing models of the Panzer III and IV.[13][14] At the insistence of General Heinz Guderian, a special tank commission was created to assess the T-34”
They were good enough that they completely outclassed the Panzer III and IV that were sent against them at the start.
"The Panzer IV was partially succeeded by the Panther medium tank, which was introduced to counter the Soviet T-34, " https://en.m.wikipedia.org/wiki/Panzer_IV
“a direct reaction to the encounters with the Soviet T-34 and KV-1 tanks and against the advice of Wa Prüf 6.[Notes 1][12] The T-34 outclassed the existing models of the Panzer III and IV.[13][14] At the insistence of General Heinz Guderian, a special tank commission was created to assess the T-34”
https://en.m.wikipedia.org/wiki/Panther_tank
https://en.wikipedia.org/wiki/T-34?wprov=sfla1
Another fun bit