Where to compare dof between the 6D and 1D4?

I guess I thought you were talking about vignetting or soft corners, not overall detail.

Sure, and those occasions are when the FF camera is cropping to the same framing, the crop camera has significantly more pixels, and/or the crop camera is using a lens that is sharper by at least a factor of the crop factor.

Shooting FF in crop mode is merely an operational convenience -- it does not maximize the IQ that the FF camera can obtain.

Like I've said from the beginning, so long as the FF sensor is at least as efficient, has at least as many pixels, the FF photo is not being cropped to the same framing as the camera with the crop sensor (or, if it is, the FF sensor has the same, or greater, pixel density than the camera with the crop sensor), and the crop camera is not using a lens that is 1.5x sharper than then FF camera, then crop will never have an IQ advantage over FF.

Or, more generally, with the exception of extreme macro or extreme telephoto shooting where the FF photographer must crop heavily, it is all but unheard of that a crop sensor camera will have higher IQ than a FF camera of the same generation (although there are certainly exceptions -- the D7000 has way more DR than the Canon 5D2, or the 5D3, for that matter).