It's not a rack, it's an aerial deflector, also called an antenna deflector. It's a little doohicky to get the radio antenna out of the way when the turret is traversed. Later models of the Pz IV moved the antenna to the rear where it would be out of the way, barring a full turret rotation, and the deflector became more or less irrelevant. Very early Pz IVs didn't always have these deflectors, but they start appearing pretty regularly with the Ausf. C. Similarly, the Pz III's radio antenna was far enough back that it would not interfere with the regular operation of the turret, so Pz IIIs didn't have aerial deflectors. What Pz IIIs and IVs both had, however, were similar little wooden strips on the bottom of the rear turret stowage bins (at least once the stowage bin was introduced). The wooden strips accomplished a similar purpose, to move the aerial out of the way when the turret traversed and (being wood) to prevent the antenna from shorting out upon contact with metal.