The wire wheel axles are dimensionally the same for all models - if you measure from the outer edge of the centre caps - my narrow bodied 4/4 and Roadster measure exactly the same.
As Peter said above, the difference is in the wheel offset. The 4/4 wheels have a protruding centre cap, the Plus 4 has the centre cap roughly in line with the rim edge, and the Roadster centre caps are recessed inside the rim.
The alloy wheel axle doesn't have the splined hub bolted on, so assuming its the same width drum to drum, alloy wheels will fit slightly closer to the drum, so a narrower track than wires. This is why the 3.7 Roadster with alloys is the same width as a Plus 4.
