Originally Posted by Bob Miller
While 100mm doesn't exactly equal 4" it seems that they interchange both terms for the same wheels.
4" is 101.6 mm... or 100 mm is 3.937". That's not nearly close enough for me, especially in a wheel/hub system which is centred by the studs, not by a precise match of wheel centre bore to hub. I suppose it would be close enough for some.
In practice, it probably doesn't matter, since there will be few if any 4x100mm wheels available with near-zero offset for a typical trailer, and similarly few if any 4x4.00" wheels available with the high offset needed for most cars which should be using the 100 mm pattern.