Starting with 4 gallons (more than 16 litres) of "very good brandy" is an enormous amount for personal use. Was the original recipe aimed at businesses?
I've found that with old alcohol recipes, they always specify a recipe to make massive amounts.
A few years ago I was looking at recipes for brewing from the Household Cyclopedia of 1881 [1]. The recipes specify malt by the sack. 3 sacks of malt to brew 3 barrels. I never did actually brew any beer from the book, since the recipes were fairly standard and not altogether that different from modern brewing. 1881 is late enough that we had a good understanding of the chemistry and microbiology involved in brewing beer.
The book also has recipes for various liqueurs, usually with recipes calling for 4 gallons of spirits. There's even a recipe for "port wine" that calls for 100 gal of claret and 12 gal of honey.
The original idea behind this kind of drinks, at least for italian "cordiale" offered to troops as part of their military daily rations[0], was a way to get rid of cheap grappa that was unsuitable for selling.
Note that grappa is made out of pomace[1], which is a compost of all the leftovers from winemaking (such as seeds, pulp, stems and must), therefore pretty cheap to make.
