I'm concerned as part of my reasoning in wanting to buy a 1D IV is the ability to make use of the significantly cheaper 300 vs buying the 400.
Is it incorrect then that the 300 becomes effectively a 390 on a 1D IV?
In terms of what you see in the image, the field of view, a 300 on a 1D IV will be the same as a 390 on a 5DIII.
In terms of the detail you will be able to resolve, a 300 on a 1D IV will be the same as a 330 on a 5DIII.
Let me give a simple example to make this clear. Suppose you have a FF with a 20 megapixel sensor, and a 1.6x crop camera with a sensor which is the same sensor as the FF but with a mask around the outside to make it 1.6x narrower and 1.6x shorter. As the crop sensor is 1.6x1.6 = 2.56 times smaller in area, it will have about 8 megapixels. If you stand the same distance away from the image with a 300mm lens on each camera, the image on the crop will be 1.6x smaller in height and length. So it will cover the same field of view as a 480mm lens. But, the cropped image that it does have will be of identical quality to that on the FF. So, in terms of resolving power, it is only as good as a 300mm lens. So, an 8 megapixel 1.6x crop APS-C has the same resolving power as a 20 megapixel FF, but the field of view is 1.6x narrower.
The ID 1V is a 1.3x crop so its field of view is 1.3x smaller than FF. But, because it has fewer megapixels than the 5DIII, it has only 1.1x better resolving power.