The problem with samples of this shape and size is correction factors for the results you obtain. We use the factors contained in a book by Haldor Topsoe Semiconductor Division. There is a section dealing with samples of the shape and size you describe, but it is rather complex. It seems to me that if you used a 25 mil spacing probe the results of measuring as shown in the diagram would be valid. Our smallest spacing probe is 0.5mm so the window needed could be 2mm.
Further factors are available from literature.
The signals can be from microvolts to hundreds of mV dependant on the material. You can't really recognize what the material is, because the raw material could be 'doped' to give almost any value of resistivity. I the current flowing between the outer two probes and V the measured voltage between the centre two probes. It can be shown that the correction factor for this is near unity, when measuring at the centre, and that the formula remains valid for measurements up to within 20 x s of the periphery.