Some after market products were made to backdate Tamiya's Ausf D, but as the quality of that kit has been well surpassed, the resin items have not been continued.
If you are looking to backdate one, there is not a substantial amount of work required to scratch build the required parts, although a little skill will be required in some areas. For a reasonably priced 'one stop' reference Squadron Signal's "PzKpfw IV in action' lists out the main visual changes and has diagrams of the turret fittings , mantlet changes, and forward vision port plates that need to be altered. Of course further references would flesh out more of these changes.