Sounds like you’re describing something like the Monowalker

There are some people who have done DIY hiking trailers by chopping up youth bikes too so maybe look into that as well.

Given that a four wheel design spreads the weight to four wheels instead of just two, four wheels can obviously move more weight more easily than two.

Gonna have to disagree just on this point. Unless you’re worried about sinking into soft ground or something, two wheels can take the weight just as well as four. Fewer wheels is often more maneuverable and there’s less friction. I mean, you specifically mention wheelbarrows which show this trade off; stability for easier rolling/maneuverability.