Well I know something about panzers me too. I dont' know for sure if a model i existed, but clearly it's not a "G" late or no late model, nor an "H" because of the reasons you point it out. The gun cache (top) it's same as ausf j model, but the turret seems to be a little different. So it's not a J model either. So, if it's not an i (??) model, I don't know what it is... Maybe Pz IV ausf. RELIC. A new Panzer IV type.
A Model I did not exist, I can assure you of that. The turret is 100% identical to the OKW version; they are the same exact model, just different units. Imo a late Model G is the closest match for a Panzer IV with the L/48 gun but no standard Schurzen.
As far as the Model J goes, the Panzer IV is technically all wrong. Off the top of my head a Model Jshould lack the view ports on the side and turret, have 3 return rollers, and simplified exhaust (See Ostwind / Sturmpanzer)