Hi and welcome to AA.
Since your tank is only 4 weeks old, you wouldn't be cycled yet. Unless you're using "seeded" bio media from an established tank?
Goldfish are known to be messy fish and have a huge bioload, meaning toxic ammonia levels would rise fairly quickly. You do have the benefit of a large tank so that would at least allow an extended time before ammonia gets to dangerous levels. You should invest in a good liquid test kit to monitor ammonia, nitrite, and nitrates so you can know when its time to do partial water changes. Once your tank develops the necessary beneficial bacteria to complete the cycle, then you'll be able to do less frequent partial water changes .
You should read this article about fish-in cycling.