BUT...taking an actual resistance measurement from the multimeter says RDS = 4 ohm!
From the readings in the OP, Vds = 3mV, Ids = 90mA, so Rds(on) = 0.03 ohm, not 4. That multimeteris not working on low ohms (most don't).
In the first post:I connected the motor between 6V and D.S was tied to ground.G was varied from 0-5V by the power supply.And I took measurements when the motor was ON.As a new experiment:I took the motor off and tried to just measure RDS without anything connected to it.At VGS = 5V ---> RDS = 0.18 ohmSo it isn't exactly close to 0.035 ohm either. Yet closer than before.
I'm betting that's mainly the resistance of your multimeter leads - you need to use a 4-terminalmeasurement (Kelvin connection) for low resistance values - that takes 2 multimeters.
Is this how you normally do a Kelvin Measurement?
One quick way to find out is 'google'. Eg. google : 'four wire resistance measurement'.