I bet "cold"/nominal is 4.7 ohm, since 5 ohm (nominal) NTC are very common in that range power.
FWIW I use 5 ohm ones in my 300W Bass amplifiers , 220V mains.
That you measure 5.7 ohm on your "good" one is reasonable, and may be due to more than one cause:
* you have not substracted multimeter "zero ohm error" from your measurement, which would make your NTC actually a 5 ohm one.
* nominal resistance is at 20C , sometimes 25C ... what is your ambient temperature?
And even holding it in your hands can change that value, , so not big deal, 4.7 ohm is the value then.
* I bet "20" means the diameter in mm, which is an indirect way to rate power dissipation ... for an accurate estimation we would need the pertinent datasheey, but I doubt we are too far from proper value.
*Mouser stocks 4.7/5 ohm 20mm diameter NTCs, I buy mine there.