Really for four adult single-tail goldfish, I'd want a 100 gal or larger pond. Single tail goldfish (and even the fancy varieties) are actually very active fish. Unlike betta which tend to lounge around, koi and goldfish are very active swimmers. If you're going to enlarge it to 60 gallons, why not dig a few more feet and go all the way to 100 gallons? When in doubt with a pond, go larger.
One last thing I forgot to mention is the depth of the pond. If it's shallow (less than a foot) you will probably need to remove the goldfish for the winter. When digging a pond you want it at least a foot below the average freezing depth for your area. In my town this is only 2.5 feet deep, however in a place like upstate New York, it's closer to 4 feet deep. When you are redigging the pond, this is something to think about