Alternator health check
It looks like your 3 field windings are good, I am guessing that 4 is an exciter but on the units I have worked on this is normally a separate coil with a permanent magnet. To make sure you really should use a megger (high voltage ohm meter) from each leg to ground, the windings look like they have been quite hot or are just very dirty. You would want a minimum of 10 Meg ohms at 500v, a 1000v would be better. If the motor is in good shape +300M ohms at 1000v is common. If the unit has been in a damp area and reads between 1 and 10 mega ohms. At 1000v it may needed to be dried out a small electric heater blowing warm air through the motor can dry things out but keep the temp below 150F to be safe some books say up to 225F but I have overheated windings at this heat. Note it may take a couple of days but if you recheck and see the resistance to ground increasing let it keep going. I have recovered up to 200 hp motors doing this that started out at approx 5 meg breaking down at 700v after 3 days I had 2.1 Gig at 1000v.