Jandel probes are quite precise puppies, they employ jewel busings/bearings, precisey machined tip radius and tip distances, they give a certiciation for each probes, with the actual pin distance. The difference is in the order of few um-s from the ideal! indeed, if you aim accurate measurements the mechanical properties of the probe becomes also very important...
Signatone probes (that I know of) are much-much simplier, I would dare to say even "primitive" compared to Jandel's, therefore I expect them to be much cheaper as well.
KLA-Tencor is a major player in the industrial 4PP measurement scenery, in older times they rolled their own probes, but as far as I know they also movedl to customized Jandel probes.
Search the internet for "ASTM-F84" or "SEMI MF84-0307" this is the standard way of performing 4PP measurements.
Practically using a real current source on the outer pins can often cause problems, especially if the pin contacts are uncertain, rapid jumps in the voltage (to keep the current constant) often have adverse effects eg. welding the pins. Instead a constant voltage source is a better approach where you monitor the current flowing.
You can build your own 4PP measuring circuit surprisingly cheap, even on an Arduino shield
For non standard sample shapes the so called "Van der Pauw" method can also be used but it is rather time consuming to faithfully perform...
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