I think the 4-wheel cars are a throw-back to the 30's, in an attempt to suck in a younger, less dextrous demo.
Jon
I would think the Thomas-type 4-wheeled cars would be a better choice than slapping euro wheels on a North American style freight car.
I think it's also a throwback to the short-lived, early MPC Mini-Max boxcars and to a lesser extent, the plastic Marx 2-axle cars (although Marx did make an attempt to simulate a 4-wheel truck design with the molded in trucks/fake wheels).
Don't foget the 4-wheel Marx 6" tin meatloaves during the days of yore.
These sets are targeting the same market that buys the various wooden trains. Small trains for small kids. Most kids that get these wouldn't care if it's got 4 or 8 wheels.
Rusty