square back alternator

More resistance is safer. Worst-case it will not charge your battery as well, but you can monitor that (if dash ammeter works). To make it go "full field", apply 12 V to the field wire (disconnected from Vreg) and see how many amps output you get.

It is hard to measure 4 ohms accurately. Always, first short the multimeter leads together and subtract that from future readings. Perhaps the armature has more turns of wire. It is the Amps x turns product that determines the magnetic field.