I was watching a 1956 English movie* about sports car racing in Italy. In the background of one scene I saw this;
italian-longjohn.jpg
Looks like a wheelbarrow wheel and dustbins. Small wheel under the load instead of way out front. Despite it's small diameter, a tire that wide should roll OK at modest pressures if it's not too stiff. A trailer suits my needs better, but if I were making a cargo bike I'd look into this idea.

Here's a 2 ply that might do the trick. At 4" width, 30 PSI would probably be overkill. Looks like Nominally 4" wide tires are typically about 3.5" as 4" tires on a 6" wheel have a 13" diameter. I googled 1 ply and found several offered from Chinese vendors with large minimum orders. Should be some available here.

*"Checkpoint", on Netflix streaming. Dumb plot but beautiful cars. Back before race cars were covered in sponsor's decals and fitted with wings to glue them to the road. Lots of good footage from the Mille Miglia with cars going fast on dangerous mountain roads. No seatbelts or any other concessions to safety other than "pudding bowl" helmets.