I've kept Goldfish for many years and they do not realy eat that much plants.. Avoid soft tissue plants.. For example true aqautics like Elodea will be eaten.
But the more harder tissue plants as mentioned above Vallis is left alone, Hornwort also is a good plant and most Potamogetons too. They are more foraging fish concentrating on the substrate.
They actualy make great pets to watch if you give them more than just a straight forward cubic volume to swim back and forth. I never kept fancy gold fish, i know they lak the agility of normal goldfish, but by character it stays a goldfish and they are very social and very inteligent and like to interact with their surroundings. Give them room to play with broken lines of sight and you will be rewarded with very playfull fish. They are very aware of beeing in a group and like to challange eachother, if one is up to something others will curiously follow.
I keep them in tubs in the summer outdoors and winter indoors.. I always build a sort of Hunnebed structure for them with cobble stones with flat ones on top and sufficiently large openings to explore and play under, over and around it.
In a 4 feet aquarium a setup like this can be realy interesting and give you a very natural display of their curious behaivor.. Covering the top stones with mosses and patches of epiphytic plants like Java fern. Than if it is an open top tank and you can build some height with larger flat slaps for example 10 to 15 cm under the surface with some moss covered rock or maybe wood, maybe even plants emersing the tank. They will also go around that they love to explore shallows as well giving a great contrast viewed from above.
Anyway, if you can give them something like this, you will be rewarded with a goldfish show you've never seen before..
As said, Fancy goldfish lak this agility, thus it should be taken into account and give them a bit more room..