Those were the supply versions of the Mark IV, as mentioned in the Wikipedia article. Do any of these survive? (Trying to look this term up on Google of course proved hopeless.) It's hard to imagine how this vehicle could have been modified to carry supplies, but my curiousity has been piqued.

If you're referring to the ammo carriers for the Karl mortar, see the two 1/35th scale kits now offered by Trumpeter. These were manufactured as such at the same time as the mortars.
Most Panzer IV tanks remained configured as battle tanks throughout the war, as the long-barreled versions were still viable combat vehicles even in 1945. Many Panzer III tank chassis were rebuilt as recovery vehicles, though, and turretless Panzer I and II chassis were still seen as ammunition carriers in 1945.
There was a proposed amphibian supply vehicle (comparable to the DUKW in function, though fully enclosed) which used Panzer IV chassis components, but it was only built tiny numbers, and the unarmored body was totally different.

I'm sorry I was unclear earlier. I meant the British Mark IV tanks from WW1.

AKA Supply tanks - the main modification was replacing the sponsons with metal boxes similar in shape to the Male sponson, but without the opening for the gun. They sometimes also had a hatch added to the roof of the cab, as the manhole further back on the hull roof could be blocked by stores. They could also tow sledges to increase the amount of material transported. The big giveawy was having SUPPLY painted on the sponsons in big white letters.

The Osprey New Vanguard on the Mark IV has a colour plate showing a supply tank.

David