I have three very active goldfish (two Orandas and one Ranchu), but the
fourth (another Ranchu) likes to hang out by itself in the corner of the
tank. When fed, it'll join the others, otherwise it'll just hang out in the
corner. Is there anything wrong or do some fish prefer to be by themselves?

I just added another fish today (total of 5 in a 55 gallon tank) and the new
fish gets along with the three active ones.