The reason we concluded we needed to do 4-wire measurement was because the fuse's resistance is very small. In this case, the fuse's datasheet states that is expected to have a resistance of ~0.9 - 1.3 mOhm, so we were thinking we had to do 4-wire measurement to accurately measure the fuse's resistance because it is so small, this would eliminate the error introduced by the resistance of the voltmeter and ammeter leads, had we just put a current through it and tried to calculate resistance that way. Our input voltage will be ~12 - 17 V and I want to test them at 1 A, 50 A and 120 A over a temperature range of 70 °C - 100 °C. So if the input current was 1 A and the fuse resistance is ~1 mOhm, then I would need a DAQ with an ADC resolution to measure well below 1 mV. Better ideas?
Using a current source plus voltmeter is exactly what a 4-wire measurement from a DMM does. If you wire multiple fuses in series, the current will be equal for every fuse as long as the input impedance of your voltmeter is much larger than the impedance of the fuse (should not be difficult) (KCL). Series resistance in the wires to the current source are irrelevant assuming it has sufficient compliance to overcome the resistance. If you use separate wires/traces for the voltmeter (Kelvin connections), series resistance in the connection to the voltmeter should not matter either (again, assuming sufficiently high input resistance). I agree with daqq that this could significantly reduce the switching complexity (since you only need to switch two terminals, and each terminal can be used for two measurements).
If you would wire five fuses in series and connect a current source across the two ends:
--[---]---[---]---[---]---[---]---[---]--
| | | | | |
A B C D E F
Then terminal 1 of your (differential) voltmeter would only have to connect to nodes A, C and E. Terminal 2 would have to connect to terminals B, D and F. So two three-pole single-throw switches (or six channels in a relay matrix) would be sufficient in this example, instead of a five-pole quadruple-throw switch (or 20 channels in a relay matrix).
As for the low voltage drop, this problem exists regardless of how you source the current (the only way to change it is to increase the current). So you need a sufficiently sensitive voltmeter regardless (be it a DMM with external mux or DAQ, possibly with an external pre-amplifier).